British Prime Minister Theresa May said Tuesday that Western allies should wait for more evidence the Syrian government carried out a chemical attack before responding with a military strike.

In a call with President Donald Trump, May told President Donald Trump she backed an international response to the suspected poison gas attack, but wanted to confirm the Syrian government’s culpability before taking action.

May’s position on the issue was laid out in a readout of the call released by the prime minister’s office, reports the Daily Mail. A similar readout from the White House did not mention May’s desire to wait for more evidence.

“President Donald J. Trump spoke today with Prime Minister Theresa May of the United Kingdom,” the White House summary stated. “Both leaders condemned Syrian President Assad’s vicious disregard for human life. The President and Prime Minister agreed not to allow the use of chemical weapons to continue.”

Trump, May and French President Emmanuel Macron have publicly agreed to coordinate a response to the “barbaric” attack, in which at least 60 people were killed and hundreds more injured. However, May is under domestic pressure to wait for lawmakers to approve Britain’s participation in a strike, and members of parliament don’t return from Easter recess until Monday.

British lawmakers have approved air strikes on Islamic State targets in Syria but not on the Russian-backed regime forces. The House of Commons voted against striking Syrian President Bashar al-Assad over his use of chemical weapons in 2013.

Though he has been coordinating with France and Britain, Trump does not appear inclined to wait for unanimous agreement on a military response to the attack. On Tuesday, the president promised to attack Syria and warned Russia against intervening to defend Assad.

“Russia vows to shoot down any and all missiles fired at Syria,” Trump said on Twitter. “Get ready Russia, because they will be coming, nice and new and ‘smart!’ You shouldn’t be partners with a Gas Killing Animal who kills his people and enjoys it!”

Trump’s rhetorical escalation came a day after Organization for the Prevention of Chemical Weapons, an international watchdog, said it would deploy a team to Syria to investigate the suspected attack. The OPCW conducted an investigation into alleged use of chemical weapons by the Syrian government in 2017, but did not have access to the attack site.