Putting fellow Republicans in the hot seat, President Donald Trump called for speedy and substantial changes to the nation's gun laws, then criticised lawmakers in a White House meeting for being too fearful of the National Rifle Association (NRA) to take action.

In a freewheeling, televised session that stretched for an hour on Wednesday, Trump rejected both his party's incremental approach and its legislative strategy that has stalled action in Congress.

Giving hope to Democrats, he said he favoured a “comprehensive” approach to addressing violence like the shooting at a Florida high school earlier this month, although he offered no specific details.

Instead, Trump again voiced his support for expanded background checks. He endorsed increased school security and mental health resources, and he reaffirmed his support for raising the age to 21 for purchasing some firearms.

Trump also mentioned arming teachers, and said his administration, not Congress, would ban bump-stock devices that enable guns to fire like automatic weapons with an executive order.

“We can't wait and play games and nothing gets done,” Trump said as he opened the session with 17 House and Senate lawmakers. “We want to stop the problems.”

Trump also raised eyebrows by suggesting that law enforcement officials should be able to confiscate people's firearms without a court order to prevent potential tragedies.

“Take the guns first, go through due process second,” he suggested.

The president has previously backed ideas popular with Democrats, only to back away when faced with opposition from his conservative base and his GOP allies in Congress. It was not clear whether he would continue to push for swift and significant changes to gun laws when confronted with the inevitable resistance from his party.

Still, the televised discussion allowed Trump to play the role of potential dealmaker, a favourite for the president. Democratic lawmakers made a point of appealing to the president to use his political power to persuade his party to take action.

“It is going to have to be you,” Sen. Chris Murphy, D-Conn., told Trump.

Trump's call for stronger background checks, which are popular among Americans, has been resisted by Republicans in Congress and the NRA.

Republicans have instead been leaning toward modest legislation designed to improve the background system already in place. Trump made clear he was looking for more and accused lawmakers of being “petrified” of the gun lobby.

“Hey, I'm the biggest fan of the Second Amendment,” Trump said, adding that he told NRA officials it's time to act. “We have to stop this nonsense.”

The White House meeting came amid fresh public debate over gun laws, fuelled by student survivors of the massacre at Florida's Marjory Stoneman Douglas High School, who have been meeting with lawmakers on Capitol Hill.

Gun legislation has lost momentum in Congress as Republican leaders showed little interest in pursuing stricter gun control laws.

Democrats said they were concerned Trump's interest may fade quickly. After the meeting, Murphy told reporters: “I'm worried that this was the beginning and the end of the president's advocacy on this issue. The White House has to put some meat on the bones. The White House has to send a proposal to Congress.”

The White House is expected to reveal more on the president's plans for school safety later this week. That announcement will likely include goals for background checks and bump stocks, though whether age restrictions will be specifically addressed remains unclear, according to an administration official who sought anonymity to discuss private conversations.

Trump rejected the way Republican leaders in Congress have framed the debate, saying the House-backed bill linking a background check measure with a bill to expand gun rights by allowing gun owners to carry concealed weapons across state lines was not the right approach.

The concealed carry measure is the gun lobby's top legislative priority. But “you'll never get it passed,” Trump told lawmakers, reminding them that Democratic senators, including some in the room, strongly oppose it.

Instead, he suggested Republicans should focus on the background check bill, then load it up with other gun control and safety measures.

Ever the marketer, Trump suggested that the leading bill adjusting the National Instant Criminal Background Check System now known as “Fix NICS” could use a new name. “Maybe you change the title, all right? The U.S. Background Check Bill, or whatever,” Trump said.

The hour-long meeting with lawmakers was reminiscent of one in January on immigration, when he told lawmakers to come up with a good bill and he would take the “heat” from critics.

That effort, however, ended in failure in Congress amid Trump's shifting views and priorities in the debate.

Among those at the White House Wednesday were Sens. Pat Toomey, R-Pa., and Joe Manchin, D-W.Va., who are pushing their bill it failed twice in the Senate after the 2012 Sandy Hook Elementary School shooting to broaden background checks to include pre-purchase reviews at online and gun show sales.

Trump asked Toomey if his plan to expand background checks included raising the minimum age for young people to buy an assault weapon. Toomey told the president it did not.

“You know why,” Trump scoffed. “Because you're afraid of the NRA.”

In fact, Toomey is one of the most high-profile Republicans on gun legislation, and the bill was opposed by the NRA. After earlier votes on the bill, the group downgraded its rating of the senator as he ran for re-election.

The meeting came after one major retailer, Dick's Sporting Goods, announced it was halting sales of assault-style rifles and high-capacity magazines at all of its stores and banning the sale of all guns to anyone under 21.

Walmart, the nation's largest retailer, followed late on Wednesday, saying it would no longer sell firearms and ammunition to people younger than 21.

The discussion was billed as a session focused on “school and community safety,” and two of those attending, Sen. Orrin Hatch, R-Utah, and Rep. John Rutherford, R-Fla., have proposed new federal grant funding to stem school violence.

The bill would offer money for law enforcement and school staff training, campus infrastructure upgrades and mental health resources.

Students make emotional return to school

With tears, fears and defiance, students also made an emotional return on Wednesday to their Florida high school where a former classmate went on a shooting rampage two weeks ago, killing 17 people.

Students at Marjory Stoneman Douglas High School in Parkland were greeted by heavy security and scores of well-wishers as they returned to classes.

Dozens of police officers lined the sidewalks saying “Good morning” to each child and retired officers passed out flowers. Former students, neighbours and their children held banners reading “We Love You,” “You've Got This” and “We Are With You.”

“It's all a little overwhelming,” said one 17-year-old student named William, who shared a classroom with two of the young victims, Nicholas Dworet and Meadow Pollack.

“It was just sad to go back there and not have my friends who were in the class with me anymore.”

Likewise, for Kimberly Miller, the first day back meant confronting the absence of her geography teacher, 35-year-old Scott Beigel.

Beigel was one of three staff killed, along with 14 teenagers, when former student Nikolas Cruz entered the school on Valentine's Day and opened fire with a semi-automatic rifle.

“It's pretty upsetting,” said the 14-year-old Miller. “But it was also refreshing to talk to everyone because people don't really understand how it feels, no matter how much they try to understand.”

While there were few open displays of grief, many students looked sombre, speaking in hushed tones with their eyes downcast.

Jonathan Abramchaev, 15, said it was “very emotional” to see his school again.

“Seeing all the flowers by the gate, that really hurt me,” he told AFP. “Today we were just discussing and talking out our feelings.”

'What happens when they go?'

Some said they felt reassured — if a little unsettled — by the heavy police presence.

“I'm not scared,” said Stoneman Douglas junior Sean Cummings. “I feel like it's more protected than any other school at this point.

“But it's still weird to see everybody here and all these police officers,” said the 16-year-old.

Others ─ like senior Carly Novell ─ said they were nervous to return. “I'm really scared to go in,” said Novell, who like many others wore a maroon T-shirt, the school colours.

Broward County school superintendent Robert Runcie said grief counsellors were on hand for the day.

“We're going to provide as much support as we can,” Runcie told CNN. “Students are excited. As a family, they're going to pull through it.”

A 15-year-old named Alan said the shooting left him fearful — even with his school under close watch by police.

“What happens when they go? In a couple of days or weeks when it gets back to normal? It may happen again,” he said.

'We have to act'

Since the shooting, Stoneman Douglas students have been lobbying politicians for stricter gun controls both in their home state of Florida and in Washington.

Republican lawmakers, with majorities in the US Congress and the Florida state legislature, have been cool on bringing in major reforms on the sales of firearms.

But Trump upped the pressure on lawmakers to get to work, hosting the bipartisan meeting at the White House where he raised eyebrows with his tough stance.

'Last father of a murdered kid'

Pressure is also growing on businesses.

On Wednesday, Dick's Sporting Goods, a large chain store, announced it would immediately stop selling assault-style rifles and would not sell guns to anyone under the age of 21.

Dick's CEO Edward Stack said Cruz had purchased a shotgun at one of his stores in November and although it wasn't the gun used in the shooting, the chain would no longer sell semi-automatic weapons.

“Our view was if the kids can be brave enough to organise like this, we can be brave enough to take them out of here,” Stack said.

Andrew Pollack ─ the father of 18-year-old victim Meadow, accompanied by his therapy dog Sunny ─ said he was determined to be the “face of the last father of a murdered kid.” “We need to make it that every kid in America, when he goes into a classroom, he knows he's safe,” he said.