Facing rising international pressure, the Obama administration continued its push Sunday to shore up support ahead of a vote by the Senate this week on a resolution authorizing military force in Syria.
Chief of Staff Denis McDonough vigorously reasserted the administration's arguments for the use of military force in a blitz of television appearances.
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White House officials have blanketed Capitol Hill for classified briefings with various lawmakers, showing them graphic videos chronicling the effects of the alleged chemical weapons attack outside Damacus on Aug. 21.
The administration claims more than 1,400 people were killed.
The Assad regime has repeatedly denied using chemical weapons in Syria's civil war.
Secretary of State John Kerry has been trying to drum up international support for military strikes against Assad, meeting with members of the European Union and Arab League over the weekend following last week's fractious G20 meeting.
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He told reporters Sunday that the Arab League is in agreement that Syrian President Bashar al-Assad used chemical weapons against his own people last month.
But, like EU foreign ministers, the league stopped short of endorsing military action.
Kerry also told Reuters on Sunday that the US has not ruled out going back to the United Nations Security Council to secure a resolution on Syria once UN inspectors complete their report on the alleged attack.