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The Hmeimim base in the coastal Syrian city of Tartus is under the protection of the Russian S-400 air-defence system, which has a range of 150 miles.

The U.S. has not ruled out further strikes, however doing so in future would now risk hitting one of the 30 Russian aircraft or 15 helicopter gunships as well as hundreds of personnel stationed at Hmeimim.

Washington fears a confrontation with Russia over Syria could escalate into a full-blown conflict between the former Cold War foes.

The reports came as the Organisation for the Prohibition of Chemical Weapons (OPCW), the UN watchdog, said “incontrovertible” test results had shown sarin gas or a similar substance was used in the April 4 attack.

Samples from 10 victims who had been treated in Turkey were analysed at four OPCW laboratories. British scientists had earlier presented similar findings to the UN security council.

Russia queried the conclusions, however. Maj Gen Igor Konashenkov, defence ministry spokesman, issued a statement Thursday questioning how the samples were collected and how the analysis could have been done so quickly.

Moscow and Tehran had jointly put forward a proposal to the OPCW to launch a new investigation but the move was rejected.