Nikki Haley, the U.S. ambassador to the United Nations, said Friday there should be no rush to launch military action on Syria, but that "at one point you have to do something."
"You don't rush decisions like this," Haley told reporters, adding that if there is haste "you make a mistake."
Haley spoke ahead of a Security Council meeting called by Russia to discuss the threat of US-led military action in Syria.
President Donald Trump is analyzing all the information and taking measures to avoid any unwanted repercussions, she said.
"We have to know that there is proof and we have to know that we are taking every precaution necessary should we take action," she said.
Separate analysis by the United States, France and Britain have concluded that a chemical attack took place on Saturday in the rebel-held town of Douma, said Haley.
The United States is consulting with Britain and France about a joint military response to the attack in Douma that medics and rescuers said left at least 40 people dead on Saturday.