Boris Johnson says it would be very difficult for the UK to refuse the US if it asked for support in another military strike on Syria.
The foreign secretary said MPs would not necessarily have a vote on any proposed joint action.
The US carried out a missile strike against a Syrian air base days after a chemical attack that left 80 people dead and hundreds wounded.
Syria’s President Bashar al-Assad denied his forces were responsible.
Mr Johnson claimed the Assad regime had “unleashed murder upon his own citizens with weapons that were banned almost 100 years ago”.
“I think it would be very difficult if the US has a proposal to have some sort of action in response to a chemical weapons attack,” he told BBC Radio 4’s Today programme.
“And if they come to us and ask for our support – whether it’s with submarine-based cruise missiles in the Mediterranean… in my view – and I know it’s also the view of the prime minister – it would be difficult to say ‘no’.”
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