Russia issued a sharp rebuke against the United States after the US military launched 59 cruise missiles at a Syrian military airfield, saying that the attack violated international law and would further damage relations that are already at their post Cold-War low.

"President [Vladimir] Putin considers the American strikes against Syria an aggression against a sovereign government in violations of the norms of international law, and under a false pretext," Kremlin spokesman Dmitry Peskov told reporters.

The first direct American assault on the government of President Bashar al-Assad since that country's civil war began six years ago would also negatively affect the war on terrorism, Peskov said. 

The Trump administration authorised the attack in retaliation for a chemical attack that killed scores of civilians this week. The US air strikes killed at least nine civilians, including four children.

The attack dramatically expands US military involvement in Syria and creates the possibility of a direct confrontation with Russia, which has backed Assad since 2015.

The Syrian government in 2013 agreed to surrender its chemical weapons under the supervision of the Russian government. Moscow and Washington had agreed that Russian forces would find, secure and destroy the weapons to ensure that Assad would not use them.

"Clearly, Russia has failed in its responsibility to deliver on that commitment from 2013," Secretary of State Rex Tillerson said. "So either Russia has been complicit or Russia has been incompetent in its ability to deliver on that agreement."

But Peskov said that Russia had overseen the destruction of the weapons.

"Syria does not have chemical weapons," Peskov said.

Tillerson, who is due to arrive in Moscow next week, said on Thursday that "there were no decisions or prior contacts, nor have there been any since the attack, with Moscow."

Russian troops are intermingled with Syrian forces on the battlefield and at bases. Any strike on a Syrian military target could also produce Russian military casualties.

Russian forces have advanced air-defence systems capable of shooting down US planes,

Until now, US aerial attacks in Syria have been largely focused on fighting the Islamic State, a common enemy of Russia, the United States and Assad's regime.

The US attack brought condemnation from other Russian officials.

"This is fraught with the possibility of a clash between Russia and the USA, and the consequences might could be the most severe, including military conflict," Mikhail Yemelyanov, a member of the lower house of the Russian parliament, told the Interfax news agency. "Here you can't rule out anything."

"This will have negative consequences for normalisation of the situation in Syria," said Viktor Ozerov, the head of the defense and security committee of the upper house of the Russian parliament. 

He said the attacks would give the wrong signal to Assad's opposition, and undermine the peace process. 

Peskov had told the Associated Press on Thursday that Russia's support for Assad was not unconditional, but insisted that Russia wants to see a full investigation into the use of chemical weapons in Syria before the United Nations takes any action.