The United States on Friday fired dozens of cruise missiles at a government-controlled airbase in Syria, in retaliation for a suspected chemical weapons attack on a rebel-held town that killed scores of civilians.

The Pentagon said 59 Tomahawk cruise missiles were fired from two warships in the Mediterranean at the Shayrat airfield in Homs province, targeting the base from where US officials believe Tuesday's attack in Khan Sheikhoun had been launched.

At least six people were killed in the early morning strike, according to the Syrian army, which denounced the US "aggression" as a violation of international law.

In a statement carried by the state-run SANA news agency and read on television, the military said the US strikes were done on a "pretext" of the Khan Sheikhoun attack, without the full facts being disclosed.

It also said the attack, which caused extensive damage to the base, made the US a partner of "terrorist groups".

Syria's state-news agency SANA gave a death toll of nine civilians, including four children.

READ MORE: Donald Trump's full statement on Syria missile strikes

It was the first direct military action the US has taken against Syrian President Bashar al-Assad's forces in the the country's long-running conflict, now in its seventh year.

"There can be no dispute that Syria used banned chemical weapons, violated its obligations under the chemical weapons convention and ignored the urging of the UN Security Council," US President Donald Trump said in a televised statement after the strike.

Syria's opposition National Coalition hailed the US attack, saying it puts an end to an age of "impunity" and should be just the beginning of further military action against Assad's forces.

Russia, a key military ally of the Assad government, strongly condemned the strikes, saying Washington's action would "inflict major damage on US-Russia ties", according to Russian news agencies.

The Pentagon said that Russia had been notified ahead of the operation - but US Secretary of State Rex Tillerson said that Washington had "sought no approval from Moscow".

At least 86 people, including 27 children, were killed after a suspected poison gas attack on Khan Sheikhoun on Tuesday, according to the UK-based Syrian Observatory for Human Rights.

ANALYSIS BY SULTAN BARAKAT, DIRECTOR OF RESEARCH AT THE DOHA INSTITUTE: It's an important step but I doubt it being a game changer as it does not represent a major shift in US policy. We have to remember that at the heart of this, there is US president who is a populist - and this is very much about the image of having reacted to some very difficult situation. There has been a lot of pressure on him and everybody else to react to those images coming out of Syria with the children dying from the gas. Until now, the US had a very small role to play in getting Assad and other parties to the negotiating table. Now they will have to play a greater role. It has to be said that this is not the first strike that the US had conducted in Syria. They have been acting for months but they were targeting ISIL.

The attack drew widespread international condemnation and public revulsion, prompting the United Nations to pledge it would investigate it as a possible war crime.

READ MORE: Syria's civil war explained

The Syrian government denied carrying out the raid. Russia has blamed the opposition, saying a government shell hit a building where rebels were producing chemical weapons. The rebels deny this.

Turkey said samples from victims of the attack indicate they were exposed to sarin, a highly toxic nerve agent.

Syria maintains it did not use chemical weapons, blaming opposition fighters for stockpiling the chemicals.

"I stress, once again, that the Syrian Arab Army did not and will not use such weapons even against the terrorists who are targeting our people," Syria's Foreign Minister Walid Muallem told reporters in Damascus on Thursday.

At the time of the US strike, Trump was at his Mar-a-Lago estate in Palm Beach, Florida, where he is holding two days of meetings with Chinese President Xi Jinping.

Trump said the strike was in the "vital national security interest" of the US.

"I call on all civilized nations to join us in seeking to end the slaughter and bloodshed in Syria, and also to end terrorism of all kinds and all types," he said.

Al Jazeera's James Bays, reporting from Palm Beach, said: "This may be a one-off operation, but it will be quite difficult from now for Trump to get himself out of the argument over the future of Syria, the political future of Assad, the UN talks process in Geneva. The Trump administration is now at the centre of it all."

Christopher Swift, professor of national security studies at Georgetown University, said the most important question was whether the Trump administration's vision in launching the strikes was "an impulsive one or a strategic one".

"It's not clear to me, yet, whether this administration has thought through the implications of the actions they took this evening," he told Al Jazeera.

"If the president has a plan, then it will be interesting to see how that plan comes through. But if he doesn't, he may have done more harm than good."

While Friday's strike was the first direct US military action against Assad's forces, fighter jets by a US-led coalition last year killed dozens of troops fighting for the Syrian government after being "misidentified" as ISIL fighters, according to the US army.

More than 100 Assad loyalists were also wounded after the attack near the eastern city of Deir Az Zor on September 16.

Over the past 24 hours, the US-led Coalition has continued targeting ISIL positions in Syria, conducting at least 20 strikes.