Jeremy Corbyn has called for any proof that the Syrian regime was behind the suspected chemical attack in Douma to be made public.
But the Labour leader warned on Friday afternoon that any Western military strikes against Bashar al-Assad would cause “chaos” and escalate the war.
Theresa May spoke to Donald Trump on Thursday night and agreed the use of chemical weapons in Syria must not go unchallenged.
The prime minister has secured the support of her cabinet for action against Assad.
French President Emmanuel Macron has said he has “proof” that Assad’s regime was behind the attack.
Speaking to Sky News, Corbyn said if France had proof who was behind the attack they should “bring it forward”.
“If there is proof the regime did it, then the regime must be held responsible. If there is proof that anybody else did it, they must be held responsible,” he said.
Asked if he had doubts over whether the Syrian regime was responsible, Corbyn said: “Look, it’s up to us to obviously condemn, obviously do everything we can, to bring about a ceasefire and a political solution, and it’s also up to the UN as the primary actor in the international community to define who actually did it.”
Labour, the SNP and the Lib Dems have demanded a vote be held in parliament before any military action is taken by the UK.
However some Conservatives have said the government does not need to wait for Commons approval.
Tom Tugendhat, chair of the foreign affairs select committee, said military action “would not require” a vote by MPs.
Johnny Mercer, a Tory MP and former soldier, said it would be a “cop-out” to ask parliament to vote.
“We elect a government and then a Prime Minister, cognisant of all the intelligence, risks and balances to take these decisions,” he said.
British ministers have agreed it was “highly likely” Assad was responsible for the attack on Saturday – which reportedly left dozens dead, including children.
Assad, as well as his allies in Iran and Russia, deny they were behind the strike.
And the Russian military has said the attack in Syria was staged and directed by Britain.
Donald Trump has delivered conflicting messages about what action the US will take.
Having initially said a missile strike was “coming”, he later said and attack “could be very soon or not so soon at all”.
Speaking in New York, UN Secretary General Antonio Guterres warned “the Cold War is back – with a vengeance”.