"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… And if they come to us and ask for our support — whether it's with submarine-based cruise missiles in the Med… in my view — and I know it's also the view of the prime minister — it would be difficult for us to say no," Johnson told the BBC Radio 4, as quoted on the broadcaster's website.
The foreign secretary did not rule out fast-tracking such a move without parliamentary approval.
US forces hit a Syrian airbase with missiles earlier in April in response to the Khan Sheikhoun chemical weapons incident in Syria, which Washington blamed on Damascus despite the lack of a proper investigation.
The United Kingdom also rushed to blame Syrian President Bashar Assad for the chemical weapons incident, with Johnson saying that Assad's responsibility was beyond doubt. Damascus denied any involvement in the chemical attack, and the Syrian army said it did not have chemical weapons as they were destroyed under a 2013 deal, monitored by the Organization for the Prohibition of Chemical Weapons (OPCW).
The United Kingdom has been involved in Syria since late 2015, when the UK Parliament approved airstrikes against terrorists as part of the US-led coalition almost two years after voting down former Prime Minister David Cameron's motion proposing military intervention in Syria in 2013.
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