Russian Prime Minister Dmitry Medvedev has said the US air strike on a Syrian air base came “within an inch” of militarily clashing with their forces.

He said the action taken by the Americans was in breach of international law and their own internal procedures, and accused Washington of “barely avoiding combat clashes with Russia”.

In a post on Facebook, Mr Medvedev said the air strike had "completely ruined relations" between the two superpowers.

However, the US says it warned Russia one hour before it fired 59 tomahawk cruise missiles at the Shayrat air base.

Satellite imagery suggests the base that was struck is home to Russian special forces and military helicopters, part of the Kremlin's effort to help the Syrian government fight Islamic State and other militant groups.

No Russian personnel were caught up in the strike.

Syrian state media has claimed that a US missile attack on a government air base has killed seven people, including four children.

Pentagon spokesman Captain Jeff Davis said the strike deliberately avoided sections of the base where the Russians were believed to be present.

Briefing reporters on the operation, he said the US military had "multiple" conversations with Russian forces on Thursday before the strike, using a line of communication that had previously been established to prevent an accidental clash in Syria during the fight against Isis.

U.S. airstrikes on Syria, explained

Mr Medvedev's statement in full:

"That’s it. The last remaining election fog has lifted. Instead of an overworked statement about a joint fight against the biggest enemy, Isis (the Islamic State), the Trump administration proved that it will fiercely fight the legitimate Syrian government, in a tough contradiction with international law and without UN approval, in violation of its own procedures stipulating that the Congress must first be notified of any military operation unrelated to aggression against the US. On the verge of a military clash with Russia.

"Nobody is overestimating the value of pre-election promises but there must be limits of decency.

"Beyond that is absolute mistrust. Which is really sad for our now completely ruined relations. And which is good news for terrorists.

"One more thing. This military action is a clear indication of the US President’s extreme dependency on the opinion of the Washington establishment, the one that the new president strongly criticised in his inauguration speech.