Pentagon chief Jim Mattis tried to clear up doubts about the US administration's North Korea strategy on Tuesday, backing Secretary of State Rex Tillerson's effort to find a diplomatic solution to the nuclear standoff.

Defense Secretary Mattis was speaking two days after President Donald Trump appeared to undermine his top diplomat by saying Tillerson was "wasting his time" by maintaining contacts with Kim Jong-Un's regime.

State Department officials insist Trump was not criticizing Tillerson, but pressuring Kim Jong-Un's regime to agree to discuss its disarmament while a diplomatic option remains on the table.

Mattis, in testimony to the Senate Armed Services Committee, said the Pentagon stands four square behind the strategy — and singled out Tillerson for support.

"The international community... is focused on the destabilizing threat posed by North Korea and Kim Jong-Un's relentless pursuit of nuclear and ballistic missile capabilities," he said.

"The Defense Department supports fully Secretary Tillerson's efforts to find a diplomatic solution but remains focused on defense of the United States and our allies."

Tillerson has explained the strategy as one of using United Nations and US sanctions and diplomatic pressure to convince Kim of his isolation and force him to negotiate nuclear disarmament.

US officials insist publicly that they have military options to counter the threat from Pyongyang if this fails, but admit privately that these are limited and highly risky.

'Little Rocket Man'

So it was hard to square Tillerson's diplomatic push with the Trump tweets that greeted him Sunday as he flew back from meeting Chinese leader in Beijing.

"I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man," Trump wrote, using his dismissive nickname for Kim.

"Save your energy Rex, we'll do what has to be done!" he added, apparently suggesting that some kind of non-diplomatic option was back on the table.

"Being nice to Rocket Man hasn't worked in 25 years, why would it work now? Clinton failed, Bush failed, and Obama failed. I won't fail."

Kim is 33 years old and came to office in 2011.

But Trump appears to have been referring to previous US efforts to deal with the North Korean dictator's father and grandfather.

While in China, Tillerson had told reporters that he was "probing" whether the North is ready for talks on the denuclearization of the Korean peninsula.

"So stay tuned," he added.

"We have lines of communications to Pyongyang. We're not in a dark situation, a blackout. We have a couple, three channels open to Pyongyang. We can talk to them. We do talk to them."

This revelation appears to have triggered Trump's tweeted response -- which in turn sparked fresh rumors of tensions between the president and his top diplomat.

Critics seized upon the remarks to paint Tillerson as an outsider, scorned by Trump and unable to speak for the United States — most scathingly in a harsh Washington Post op-ed entitled: "Donald Trump's dog."

Mattis, however, was at pains to insist that the government is working together as one to counter its most urgent threat.

"President Trump's guidance to both Secretary Tillerson and me has been very clearly that we would pursue the diplomatic efforts," he told lawmakers.

"All we are doing is probing, we are not talking with them ... So I don't see the divergence as strongly as some have interpreted it."

"When the Secretary says "probing,' he means: we're keeping our eyes open to see how sanctions, to see how the pressure campaign is affecting that government," a State Department source said privately.

Mattis noted that Trump had sent Tillerson to Beijing to work with Chinese leaders to strengthen the common diplomatic response to the crisis.

"In fact this is part of a whole-of-government, integrated effort that we have under way right now," Mattis said. "And that's what Secretary Tillerson was carrying forward for the president."

Still, dialogue appears distant, on the State Department's end.

"At some point, of course, we would like to sit down and talk with North Korea. But now is not the time to do so; they are doing too many horrible activities," added State Department spokeswoman Heather Nauert.