President Donald Trump ordered a massive military strike on a Syrian air base on Thursday in retaliation for a “barbaric” chemical attack he blamed on President Bashar al-Assad.

The massive strike — the first direct US action against Assad's government and Trump's biggest military decision since taking office — marked a dramatic escalation in American involvement in Syria's six-year civil war.

It followed days in which images of dead children and victims suffering convulsions from the suspected sarin gas attack in the rebel-held town of Khan Sheikhun shocked the world.

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Syria's regime has denied any use of chemical weapons and state media on Friday described the US strike — which was reported to have pulverised the base and killed at least four servicemen — as an “act of aggression”.

Russia too denounced the US action, with Kremlin spokesman Dmitry Peskov saying President Vladimir Putin considered it “aggression against a sovereign state” that would inflict “considerable damage” on US-Russia ties.

Trump announced the strike in a brief televised address delivered hours after the UN Security Council failed to agree on a probe into the suspected chemical attack.

Declaring it in America's “vital national security interest” to prevent the spread of chemical weapons, Trump accused Assad of a “very barbaric attack” in which “even beautiful babies were cruelly murdered”.

“Tonight I call on all civilised nations to join us in seeking to end this slaughter and bloodshed in Syria and also to end terrorism of all kinds and all types,” Trump said.

Officials said the US fired 59 Tomahawk cruise missiles at the Shayrat Airfield at 3:40am Syrian time on Friday.

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'Blown to pieces'

The missiles were fired from the USS Porter and the USS Ross, which belong to the US Navy's Sixth Fleet and are located in the eastern Mediterranean.

The strike targeted radars, aircraft, air defence systems and other logistical components at the military base south of Homs in central Syria, from where Washington believes Tuesday's deadly strike was launched.

Officials said measures had been put in place to avoid hitting sarin gas they said was stored at the airfield. “The airbase was almost completely destroyed — the runway, the fuel tanks and the air defences were all blown to pieces,” the Syrian Observatory for Human Rights said.

The Britain-based monitoring group said at least four servicemen were killed, including an air commodore. The base was the second most important for Syria's air force, Observatory chief Rami Abdel Rahman told AFP, after the Latakia airbase in Assad's coastal heartland where Russia also maintains extensive facilities.

Homs governor Talal Barazi told AFP there were several dead and wounded at the base and that large parts of it were on fire.

“It will take some time to determine the extent of the damage,” he said.

“Of course we condemn this, all action targeting Syrian military bases is condemnable.”

Syrian state media confirmed the strike, with news agency SANA saying: “This American aggression follows the slanderous media campaign by some countries... after what happened in Khan Sheikhun.”

Iran, another key Assad ally, also condemned the action.

Opposition urges more US action

But Syria's opposition National Coalition hailed the strike and called for further US action against Assad's air force.

“The Coalition welcomes the strike and urges Washington to neutralise Assad's ability to carry out air raids,” spokesman Ahmad Ramadan told AFP. “We hope for more strikes... and that these are just the beginning.”

The strike came despite a warning from Russia of potential “negative consequences” if Washington carried out military action in Syria.

Russian has insisted that the chemical weapons that caused the deaths in Khan Sheikhun had been stockpiled by “terrorists” on the ground and possibly released by a conventional strike.

US Secretary of State Rex Tillerson accused Russia — which props up the Assad regime and agreed to mothball Syrian chemical weapons in a 2013 deal — of being incompetent or complicit in permitting Assad's actions.

Russian military officials in Syria were informed of the strike beforehand in order to avoid casualties that could prompt a broader crisis.

The White House was quick to paint the decision as limited to deterring the use of chemical weapons, and not part of a broader military campaign to remove Assad by force.

“The intent was to deter the regime from doing this again, and it is certainly our hope that this has had that effect,” Pentagon spokesman Navy Captain Jeff Davis told reporters.

Tillerson said the attack should leave no one in any doubt that Trump is willing to act if any actor “crosses the line”. It will send ripples around the world, from Pyongyang to Tehran, as nations and leaders take the measure of the novice but often bellicose president.

The timing of the strike, during a meeting with China's President Xi Jinping, will give weight to Trump's threats to deal with North Korea's nuclear and missile programmes unilaterally if necessary.

Sends a 'strong message'

Allies rushed to support the US military action including Britain, Saudi Arabia and Israel.

“In both word and action, President Trump sent a strong and clear message today that the use and spread of chemical weapons will not be tolerated,” Israeli Prime Minister Benjamin Netanyahu's office said.

On Wednesday, Trump had decried the suspected attack — which killed at least 86 people, including 27 children, and wounded more than 500 — as an “affront to humanity”.

“It crossed a lot of lines for me,” Trump said, alluding to Barack Obama's failure to enforce his own “red line” on the use of chemical weapons in Syria four years ago.

In 2013, Trump had urged then-president Obama not to intervene against Assad.

The Khan Sheikhun incident appears to have marked a turning point for Trump, just days after his administration signalled it was no longer seeking the Syrian leader's departure from power.

Tillerson called Thursday for “a political process that would lead to Assad leaving” and said his future role in the country was "uncertain".