The UK government is “waiting for instructions” from Donald Trump about whether to launch a missile strike on Syria, Jeremy Corbyn has said.
The Labour leader spoke out after Cabinet ministers agreed “on the need to take action” in Syria to “deter the further use of chemical weapons”.
Ministers said it was “highly likely” the Assad regime was responsible for a suspected chemical attack.
But Mr Corbyn said military action was unlikely to solve the situation.
Medical sources say dozens of people were killed, including children, during the alleged toxic bombing of the formerly rebel-held town of Douma, in the Eastern Ghouta region.
Downing Street said ministers at a cabinet meeting agreed that the use of chemical weapons must not “go unchallenged”.
But Transport Minister Jo Johnson later stressed: “There has been no decision to take military action at this point.”
No details of UK involvement in any military action in Syria were mentioned in the Downing Street statement.
But Mr Corbyn said: “More bombing, more killing, more war will not save life. It will just take more lives and spawn the war elsewhere.”
He added: “The government appears to be waiting for instructions from President Donald Trump on how to proceed.
“Britain should press for an independent UN-led investigation of last weekend’s horrific chemical weapons attack so that those responsible can be held to account.”
President Trump spoke to the prime minister on Thursday evening, and the pair agreed that Syrian President Bashar al-Assad had “established a pattern of dangerous behaviour in relation to the use of chemical weapons”.
A statement from Downing Street added: “They agreed to keep working closely together on the international response.”
President Trump is expected to speak to French President Emmanuel Macron about what form action could take, with the White House saying “no final decision has been made”.
BBC political editor Laura Kuenssberg said it was understood the UK was planning a genuinely joint effort with the US and France, rather than playing a supporting role.
Sources say the PM is prepared to take action against the Assad regime without first seeking parliamentary consent.
There have been calls from opposition parties and some Conservative MPs for Parliament to have a vote beforehand.
Labour’s shadow home secretary Diane Abbott told BBC Radio 4’s Today programme: “It would be outrageous for the government not to bring military action in Syria to Parliament – for Parliament to have a vote…
“The reason they’re not doing it is they are frightened they’ll lose the vote.”
She also indicated that she sees Russia, which provides military support to the Syrian regime, as “a greater threat to world peace” than the United States.
Pushed on the point by Today, Ms Abbott said: “It’s clear, that at this point, Russia – its role in Syria… its role in the poison gas attack in Salisbury – is a greater threat to world peace than the United States.”
Mrs May has said “all the indications” are that the Syrian regime of president Bashar al-Assad, which denies mounting a chemical attack, was responsible for the alleged attack in the formerly rebel-held town of Douma.
During Thursday’s cabinet meeting called to discuss the UK response, she described it as “shocking and barbaric” and said it was a “further example of the erosion of international law in relation to the use of chemical weapons, which was deeply concerning to us all”.
By Chris Buckler, BBC US correspondent
The diplomatic discussions are starting to feel like part of a waiting game.
The White House insists that final decisions have still to be made.
However, in their phone call, President Trump and Mrs May agreed it was vital action was taken to deter President Assad’s forces from using chemical weapons.
Evidence is still being gathered, but US officials are understood to have results from blood and urine samples that indicate chlorine and a nerve agent were used in the Syrian town of Douma.
But they privately admit they can’t be 100% sure.
And with Russia threatening retaliation – and warning of the risk of some kind of conflict with the US – there is pressure to develop a wider strategy, alongside any plan for military strikes.
Downing Street said the cabinet had “agreed it was vital that the use of chemical weapons did not go unchallenged” and “on the need to take action to alleviate humanitarian distress and to deter the further use of chemical weapons by the Assad regime”.
“Cabinet agreed the prime minister should continue to work with allies in the United States and France to coordinate an international response,” the statement added.
Senior figures from Russia have warned of a Russian response to a US attack.
On Tuesday President Donald Trump tweeted to warn Russia that a missile attack on Syria “will be coming”.
In a subsequent tweet on Thursday, the US president said an attack on Syria “could be very soon or not so soon at all”.
MPs are due to return to Westminster from the Easter recess on Monday – and a row is continuing between some MPs over whether a vote should take place in Parliament before any action is taken.
Conservative MP and former soldier Johnny Mercer said Mrs May had a “duty” to speak to advocate any action and “bring the country with her”.
But he said it was “intellectually bankrupt” to expect the security services to lay out all the information they have.
Lib Dem leader Sir Vince Cable did not rule out backing military action but he said Parliament would have to give its approval, with conditions.