Boris Johnson has said Britain should consider joining military action against Syrian President Bashar Assad's regime if there is fresh "incontrovertible" evidence he has used chemical weapons against his own people.
The Foreign Secretary said that while the West could not intervene to change the odds in favour of the rebels fighting the regime, he believed the use of illegal weapons should not go unpunished.
"It's very important to recognise there's no military solution that we in the West can now impose," he told the BBC Radio 4 Today programme.
"The people listening to us, listening to this programme in eastern Ghouta cannot get the idea that the West is going to intervene to change the odds dramatically in their favour.
"But what I think we need to ask ourselves as a country, and I think what we in the West need to ask ourselves, is can we allow the use of chemical weapons, the use of these illegal weapons to go unreprieved, unchecked, unpunished? And I don't think that we can."
He went on: "If there is incontrovertible evidence of the use of chemical weapons, verified by the Office of the Prevention of Chemical Weapons, if we know that it's happened and we can demonstrate it, and if there is a proposal for action where the UK could be useful, then I think that we should seriously consider it."
Mr Johnson's comments came as a five-hour pause in the regime's assault on the rebel-held enclave of eastern Ghouta, close to the capital Damascus, was beginning.
The respite was ordered by Mr Assad's chief backer Russia, which has said it would be repeated on a daily basis to allow civilians trapped by the fighting to leave.
On Saturday, the United Nations Security Council passed a resolution calling for a 30-day ceasefire, but it did not set a specific start date.
More than 500 people have been killed since last week in eastern Ghouta, where activists on Sunday reported a suspected poison gas attack.