U.S. lawmakers briefed by senior Obama administration officials Thursday say there is no doubt the Syrian government carried out a chemical weapons attack last week.
Members of Congress asked the White House and State Department to justify a possible military strike against Syria.
Democratic Representative Eliot Engel said the U.S. officials cited intercepted communications between senior Syrian officials.
Engel also said intelligence showed the Syrians moved materials around in advance of a chemical strike.
Earlier Thursday, White House spokesman Josh Earnest said that along with circumstantial evidence, the United States has classified intelligence that undoubtedly points to an attack by the Syrian government.
President Barack Obama is still deciding how to respond to Syria. A military strike against the Assad government appears to be the most likely course of action.
U.N. Secretary-General Ban Ki-moon is urging world powers to hold off any action until a U.N. chemical weapons team completes its work. The inspectors are expected to leave Syria on Saturday.
After two days of closed-door meetings, diplomats from the five permanent Security Council members have been unable to agree on a possible military strike. The Obama administration has said it may act on its own against Syria if the U.N. continues what it calls "diplomatic paralysis."
Syrian officials deny using chemicals and accuse the rebels of using such weapons on Syrian soldiers.