WASHINGTON (NYTIMES/AFP/Bloomberg) - Donald Trump authorised missile strikes against an Syrian airbase before sittitng down to dinner with visiting Chinese leader Xi Jinping at his Mar-a-Lago resort in Florida.

The operation was carried out less than an hour after the US president concluded the meal of steak and pan-seared sole with Xi, sending an aggressive message about Trump's willingness to use the military power at his disposal.

Before dinner, Trump had convened what officials described as a "decision meeting" on the strikes with his top national security aides - many of them with him at Mar-a-Lago, and others on secure video screens from Washington.

After what aides called a "meeting of considerable length," Trump authorised the strikes.

He also informed the Chinese leader personally about the missile strikes targetting Al Shayrat airfield, a White House official said.

Trump was later updated about the operation by Defense Secretary James Mattis after dinner, according to a US official, a CNN report said.

The attack came two days after Syrian president Bashar al-Assad's regime used poison gas to kill scores of civilians, an act that drew international condemnation and that Trump called "'an affront to humanity."

Traveling to Mar-a-Lago from Washington on Thursday (April 6), Trump had spoken to reporters aboard Air Force One about Assad, saying "what happened in Syria is a disgrace to humanity. And he's there, and I guess he's running things. So something should happen."

Soon after, Secretary of State Rex Tillerson appeared in a hastily arranged news conference in Florida - his first in the US since he was confirmed for the job - ostensibly to discuss Xi's visit. But the real intent was to address the Syria issue, according to a State Department official who asked not to be identified because he wasn't authorised to speak publicly about the matter.

The summit with the Chinese leader will continue Friday, and some US officials believe the strike will also serve as a warning of US willingness to strike North Korea, if China does not act to curtail the nuclear ambitions of the government there.

The two leaders are to discuss what to do about North Korea's nuclear programme and US-China trade disagreements on Friday (US time), followed by a working lunch around 12 midnight Singapore time.

According to one military official, 59 Tomahawk cruise missiles were launched from two US Navy warships. The official said that the cruise missile strike was at the more limited end of the military options presented to Trump on Thursday by Mattis. The cruise missile strike, the official said, was intended to send a message to President Bashar Assad of Syria about the US intention to use military force if he continues to use chemical weapons.

It was the first time that the White House had ordered military action against forces loyal to Assad. The speed with which the Trump administration responded - and remarks earlier in the day by US officials who said that options were still being considered - appeared intended to maximise the element of surprise.

It was Trump's first order to the military for the use of force - other operations in Syria, Yemen and Iraq had been carried out under authorisation delegated to his commanders - and appeared intended to send a message to North Korea, Iran and other potential adversaries that the new commander in chief was prepared to act, and sometimes on short notice.

Trump authorised the strike with no congressional approval for the use of force, an assertion of presidential authority that contrasts with the protracted deliberations over the use of force by his predecessor, former President Barack Obama.

Unlike Obama, who weighed - and ultimately rejected - the use of a similar strike at targets after Syria used chemical weapons in 2013, Trump moved with remarkable speed, delivering the punishing military strike barely 72 hours after the devastating chemical attack that killed 80 people this week.

Two Republican senators, John McCain of Arizona and Lindsey Graham of South Carolina, praised the strike in a statement and called for Trump to go further and to "take Assad's air force - which is responsible not just for the latest chemical weapons attack, but countless atrocities against the Syrian people - completely out of the fight."