Secretary of State John Kerry said the chemical weapons attack killed 1,429 people, of which he said 426 were children.
For now, many lawmakers were praising Obama for putting the question to Congress. But it is not expected to act until the House of Representatives returns from recess on Sept. 9.
"Under the Constitution, the responsibility to declare war lies with Congress," said House Speaker John Boehner in a statement. Boehner said his chamber would consider the question the week of Sept. 9.
"We are glad the president is seeking authorization for any military action in Syria in response to serious, substantive questions being raised," he said.
"The President's role as commander-in-chief is always strengthened when he enjoys the expressed support of the Congress," agreed the top Republican in the Senate, Mitch McConnell of Kentucky.
Rep. Peter King, however, said Obama was abdicating his role as commander-in-chief. The New York Republican suggested the president was undermining the authority of future presidents and seeking a political shield for himself by going through Congress.
"The president doesn't need 535 Members of Congress to enforce his own red line," he said.
Obama had said earlier that Assad's use of chemical weapons would cross a red line, suggesting greater U.S. intervention in the Syrian conflict.
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- AP