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.
But Russia's Foreign Minister, Sergei Lavrov, said its specialists had "irrefutable" evidence that the alleged attack had been faked by foreign intelligence agents.
He said inspectors from the Organisation for the Prohibition of Chemical Weapons "will find no proof of chemical weapons or chlorine", adding that "this was another staged event - and the staging was done by the special services of one of the countries that has been at the forefront of the anti-Russian campaign".
Campaigners from the Stop the War Coalition, an organisation once chaired by Jeremy Corbyn, handed a letter to Downing Street signed by MPs, trade unionists, celebrities and academics urging Mrs May not to take military action in Syria.
On Thursday, cabinet ministers 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."
Sources say the PM is prepared to take action against the Assad regime without first seeking parliamentary consent.
Joint effort
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."
In a later interview, the Labour leader would not say if there were any circumstances in which he would back military intervention in Syria, adding: "Let's cross that bridge when we get there."
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.
Labour's Shadow Home Secretary Diane Abbott hesitated repeatedly when asked by BBC Radio 4's Today programme whether she believed Russia or the US presented a "greater threat" to world peace.
"At this point, after Salisbury, and with the unremitting support of Russia for Assad, you have to say Russia isn't bringing forward the cause for world peace," she said.
Pushed again on the point by presenter Nick Robinson, Ms Abbott clarified: "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."
Ms Abbott also joined calls from UK opposition parties and some Conservative MPs for Parliament to have a vote before any military action is taken, saying it would be "outrageous" if the request is denied.
"The reason they're not doing it is they are frightened they'll lose the vote."
Media captionShould Parliament have a vote on Syria strikes?
But Conservative backbencher Chris Philp said he disagreed - and instead urged MPs to trust the prime minister.
"To ask me to vote on an issue as important as this without seeing all of the material, is something that one should think about very carefully," he told the BBC.
"And that's why, if there are things that can't be disclosed publicly in Parliament, there are circumstances under which you can have action without a vote first."
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".
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.
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".
Donald J. Trump
✔
@realDonaldTrump
Never said when an attack on Syria would take place. Could be very soon or not so soon at all! In any event, the United States, under my Administration, has done a great job of ridding the region of ISIS. Where is our “Thank you America?”
@realDonaldTrump
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
"NO WAR, NOT IN MY NAME!!!!