British Prime Minister Theresa May and President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE vowed in a phone call Thursday to deter further use of chemical weapons by Syrian President Bashar Assad in the wake of last week's deadly attack, according to the U.K. leader's office.

“They agreed it was vital that the use of chemical weapons did not go unchallenged, and on the need to deter the further use of chemical weapons by the Assad regime,” the prime minister's office said in a statement reported by Reuters.

“They agreed to keep working closely together on the international response."

The White House said in a readout of the call that Trump and May "continued their discussion of the need for a joint response to Syria’s use of chemical weapons."

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Trump was also slated to speak with French President Emmanuel Macron on Thursday evening as the United States and its allies weigh a response to the apparent chemical weapons attack on a rebel-held area of Syria last weekend.

The White House said Thursday that Trump met with his national security team to discuss Syria but "no final decision has been made" despite the president's tweet the previous day warning Russia to "get ready" for missiles headed to Syria, its ally.

"We are continuing to assess intelligence and are engaged in conversations with our partners and allies," White House press secretary Sarah Huckabee Sanders said in a statement on Thursday, noting Trump's calls with May and Macron.

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?” — Donald J. Trump (@realDonaldTrump) April 12, 2018

May's office announced on Thursday her Cabinet agreed to "take action" in response to the attack, though the exact response remains unclear.

Macron said he had proof Assad's government was behind the attack on Saturday, while Damascus has denied involvement in the attack.

Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE said this week that the U.S. was looking for evidence as the Trump administration weighs its response.

"I believe there was a chemical attack, and we are looking for the actual evidence," Mattis said.

Updated at 10:17 p.m.