Britain will support any fresh retaliation by the United States for the use of chemical weapons by the Syrian regime of Bashar Assad, Defence Secretary Sir Michael Fallon has said.
The White House has said it has "potential" evidence the Syrian military is preparing for another chemical attack against rebel forces and warned the regime would pay a "heavy price" if it went ahead.
Sir Michael said that Britain had backed the US administration of President Donald Trump when it mounted missile strikes against the regime following a chemical attack last April and was prepared to do so again.
"As always in war, the military action you use must be justified, it must be legal, it must proportionate, it must be necessary. In the last case it was," Sir Michael told the BBC Radio 4 Today programme.
"If the Americans take similar action again, I want to be very clear - we will support it."
Earlier, White House press secretary Sean Spicer said the US had identified "potential preparations for another chemical weapons attack by the Assad regime that would likely result in the mass murder of civilians, including innocent children".
Mr Spicer said the activities were similar to preparations taken before the attack in April on Khan Sheikhoun in the rebel-held Idlib province which killed dozens of men, women and children.
He warned that if "Mr Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price".
Sir Michael said the US had been monitoring the situation in Syria although it had not shared any specific evidence with the British Government about the regime preparations.
He said he expected to discuss the current position with US defence secretary General James Mattis when they meet at a gathering of Nato defence ministers on Thursday.
"Jim Mattis did review the various options with me last time before the final meeting with the president. I shall be seeing Secretary Mattis at the Nato meeting on Thursday. This is one of the matters we expect to review," he said.
He said the use of chemical weapons by the Assad regime against its own people was "absolutely abhorrent".
"If they use chemical weapons, what is important is to further degrade their capacity to do so.
"That is why the strike last time was very carefully limited. It was limited in time, it was limited just to the airbase to tackle the aircraft and the equipment that was dropping the weapons."
Sir Michael revealed that Britain had been using its "offensive" cyber warfare capabilities against Islamic State (IS) - also referred to as Daesh - in both Syria and Iraq.
"I am confirming today that we have been using offensive cyber against Daesh in Raqqa just as we have previously used it in Mosul," he said.
"In Mosul we had some success in support of the Iraqi forces in disabling some of the computer systems that supported the infrastructure there and weakened the capacity of Daesh to resist the Iraqi forces.
"The same is now happening over in Syria, where we have been aiding the Syrian Democratic Front in their struggle to get Daesh out of Raqqa."