Donald Trump has led international condemnation of an alleged chemical weapons attack by the Syrian government, raising prospects of an American response to the assault that has killed at least 48 people in the rebel-held town of Douma.

The US president singled out Russian president Vladimir Putin in a tweet on Sunday, saying he held Russia and Iran, Syrian president Bashar al-Assad’s backers, responsible for the atrocity. He slammed the “Animal Assad” and warned there would be a “big price to pay” for the attack.

Republican lawmakers urged Mr Trump to turn his words into action. Senator Lindsey Graham, a Republican foreign policy hawk, said a soft response from Mr Trump would damage his credibility. “If he doesn’t follow through and live up to that tweet, he’s going to look weak in the eyes of Russia and Iran,” he warned.