Theresa May has dismissed as "hypothetical" a suggestion from Foreign Secretary Boris Johnson that it would be difficult to say No to supporting further US air strikes against Bashar Assad's regime.
The Prime Minister stressed the importance of finding a solution with a "political transition away from president Assad".
Mr Johnson had suggested it would be hard to turn down a request to support Donald Trump if he sought to launch further strikes in response to any use of chemical weapons by the Syrian regime.
And the Foreign Secretary indicated that the Prime Minister might not seek MPs' approval in a parliamentary vote before British forces were deployed.
Mrs May sought to play down the prospect of military action, saying: "This is a hypothetical issue because there is no proposal on the table for further strikes in Syria.
"What is important is that we look to see how we can bring about a solution in Syria that is going to lead to a strong and stable Syria.
"That means a political transition, it means political transition away from president Assad."
On a campaign visit in Chesterfield she said the UK had been at the forefront of diplomatic efforts to say to the Syrian regime's backers, including Russia, that "now is the time to stop president Assad from bombing his own people".
Liberal Democrat leader Tim Farron said it was "critical" for ministers to obtain Parliament's consent before taking action, and warned Mrs May not to do so under cover of the election period, when there were no MPs in place.
Labour leader Jeremy Corbyn said the Government should focus on securing a political settlement in Syria, rather than taking unilateral action.
Mr Johnson said there was "no question" that Assad's regime was responsible for the sarin gas attack earlier this month which prompted a US retaliatory cruise missile strike on a Syrian air base.
Asked whether Britain would be ready to join the US in similar action if the chemical assault was repeated, Mr Johnson told BBC Radio 4's Today programme: "I think it would be very difficult for us to say no.
"How exactly we were able to implement that would be for the Government, for the Prime Minister.
"But if the Americans were once again to be forced by the actions of the Assad regime - don't forget, it was Assad who unleashed murder upon his own citizens with weapons that were banned almost 100 years ago - if the Americans choose to act again and they ask us to help, as I say, I think it would be very difficult to say no."
Britain is carrying out air strikes in Syria and Iraq against terrorists of the so-called Islamic State (IS), but has taken no military action against the Assad regime.
MPs backed the action against IS in a December 2015 Commons vote but rejected strikes against the Syrian government in 2013.
Mr Johnson said that the question of whether parliamentary approval would be needed for any military action "needs to be tested".
Pressed on whether the Government would have to bypass Parliament if there was a phone call in the night calling on the UK to support US action, he told LBC radio: "You are putting your finger on the issue."
There was no doubt about Assad's responsibility for the April 4 chemical attack on Khan Sheikhoun in north-western Syria, in which more than 80 people are believed to have died.
"We know Assad did it," the Foreign Secretary said.
"The US responded. On that occasion they did not ask us for assistance.
"If Assad were to do something like that again and if there was a response that was commensurate and appropriate and timely and all the rest of it and the US were to come to us and ask us for support and practical assistance, you would have to work out what the procedures would be but I think it would be very difficult for us as a country given our relationship with the United States for us to say no."
There is no legal requirement on the Government to seek parliamentary approval before ordering military action but it has become convention to consult MPs, except in the event of an emergency.
Mr Corbyn said: "We don't need unilateral action, we need to work through the UN, but above all we need to bend ourselves totally to getting a political settlement in Syria and allow the inspectors space to work, allow them to make sure we know who did that terrible chemical weapons attack and also recognise that the inspectors there are already destroying any stocks of chemical weapons that they find."
Mr Corbyn said Mrs May should consult other parties if she received a US request for military support while Parliament is dissolved for the General Election.
Speaking to BBC political editor Laura Kuenssberg during a campaign visit to Harlow, Mr Corbyn said: "If the president does ask for that, we're now into a period where there's virtually a caretaker government.
"Any decision made by the Government on this would have to be done in consultation with all the other parties in parliament."