British prime minister Theresa May told U.S. president Donald Trump on Tuesday, April 10 that Britain would need more evidence of a suspected chemical attack by the Assad regime before joining a military strike against Syria, The Times reports.

The prime minister rejected a swift retaliation as inspectors from the Organisation for the Prohibition of Chemical Weapons (OPCW) prepared to visit the Damascus suburb where at least 40 people were reported to have been killed by chlorine gas on Saturday.

Trump had promised on Monday that he would decide the U.S. response within 48 hours. As that deadline approached, it appeared that he too was drawing back from an imminent strike.

The US president cancelled weekend travel plans amid reports that Russian and Iranian involvement in Syria had complicated White House calculations about the response.