UNITED NATIONS, United States (AFP) - The United States on Friday (April 7) threatened further military action in Syria following its missile strikes on an air base in the war-wracked country in retaliation for an apparent chemical attack.

US Ambassador Nikki Haley delivered the warning at an emergency session of the Security Council where Russia accused the United States of violating international law and waging an "act of aggression" against Syria.

"The United States took a very measured step last night," Haley said. "We are prepared to do more, but we hope it will not be necessary."

Damascus and its ally Moscow furiously condemned the missile strike that marked the first direct US assault on President Bashar al-Assad's government.

But US allies rallied around Washington after President Donald Trump launched the massive strike in retaliation for a "barbaric" chemical attack he blamed on Assad.

Haley said the strikes destroyed an air field from which the United States believes the attacks on the rebel-held town of Khan Sheikhun were launched, killing 86 people including 27 children.

"We were fully justified in doing so," she said.

"The United States will no longer wait for Assad to use chemical weapons without any consequences," Haley said. "Those days are over."

Assad's office called the strike "foolish and irresponsible" and Moscow announced a series of retaliatory steps including plans to strengthen Syrian air defenses.

The United States fired 59 Tomahawk cruise missiles from warships in the Mediterranean at the Shayrat airfield, dealing heavy damage to the base near Homs in central Syria.

"The United States attacked the territory of sovereign Syria," Russian Deputy Ambassador Vladimir Safronkov told the council.

"We describe that attack as a flagrant violation of international law and an act of aggression." The United States did not seek Security Council authorization for the military action that followed days of global outrage at images of dead children from the suspected sarin gas attack.

It was Trump's biggest military decision since taking office and marked a dramatic escalation in American involvement in Syria's six-year war.

US-Russia ties

Syrian state news agency SANA said nine civilians including four children were killed in villages near the base.

"What America did is nothing but foolish and irresponsible behavior, which only reveals its short-sightedness and political and military blindness to reality," Assad's office said.

While threatening further strikes, the US envoy also said it was time to press on with diplomatic efforts to achieve a political solution to end the war.

"Now we must move to a new phase: a drive toward a political solution to this horrific conflict." Damascus has denied using chemical weapons in Khan Sheikhun.

With US Secretary of State Rex Tillerson due in Moscow next week, the Kremlin warned the US military action would inflict "considerable damage" on US-Russia ties.

It immediately suspended a deal with the United States aimed at avoiding clashes in Syrian airspace.

Foreign Minister Mohammad Javad Zarif of Iran, another staunch Assad ally, tweeted that the strike was based on "bogus CW (chemical weapons) allegations" and would aid jihadists like the Islamic State of Iraq and Syria group.

Britain, Canada, France, Germany, Israel, Japan, Saudi Arabia and Turkey all supported Washington, with Ankara also calling for a no-fly zone in Syria.

UN envoy Staffan de Mistura announced the UN-backed ceasefire taskforce on Syria that is co-chaired by Moscow and Washington would meet later on Friday at Russia's request.

'End this slaughter'

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

"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," he said.

The missiles, fired from the USS Porter and the USS Ross, targeted radars, aircraft, air defense systems and other logistical components.

Russia's military said the strike had an "extremely low" military impact, with fewer than half of the 59 missiles reaching the air base.

It destroyed six planes under repair and several buildings, including a storage depot and radio station, it said.

US officials said Russia's military in Syria was informed of the strike beforehand in order to avoid casualties that could prompt a broader crisis.

A Syrian military source said its armed forces had also been warned in advance, without specifying by which party.

"We took precautions in more than one military point, including in the Shayrat airbase. We moved a number of airplanes towards other areas," the official told AFP.

'Enough killing and injustice'

Russia has stood by Damascus despite the global uproar, insisting the chemical weapons that caused the deaths in Khan Sheikhun had been stockpiled by "terrorists" and possibly released by a conventional strike.

In Khan Sheikhun, residents still mourning their dead welcomed the US strike as a way to pressure Damascus.

"God willing, these strikes will be a clear warning to Bashar al-Assad, to tell him: Bashar, enough killing and injustice against these people," said Abu Ali, a man in his 40s.

Opposition and rebel fighters, who have for years urged more direct US military action in support of their uprising, hailed the strike and called for more.

The National Coalition, the main opposition grouping, urged Washington to "neutralize" the regime's air power while the Free Syrian Army and other rebel factions called for more strikes to "prevent the regime from using its airports and internationally-banned weapons against Syrians".

Turkish President Recep Tayyip Erdogan, a longtime foe of Assad, also called for more "steps for a serious result to protect the oppressed Syrian people." The White House has until now painted its decision as limited to deterring the use of chemical weapons, and not part of a broader military campaign.

Trump had previously indicated no willingness to engage further in Syria's civil war, beyond stepping up efforts to battle the jihadists of IS, who have been targeted by US-led air strikes in Syria and Iraq since mid-2014.

His administration had recently signalled it was longer seeking Assad's departure.

Since Khan Sheikhun the United States has dramatically changed tack, however, with Trump warning that the "barbaric" attack required a response, and Secretary of State Rex Tillerson calling for "a political process that would lead to Assad leaving."