President Barack Obama, under pressure from some lawmakers to provide more help to Syria's opposition, asked Congress on Thursday to approve $500 million to train and equip moderate Syrian rebels seeking to oust President Bashar Assad.

It was the administration's most tangible move yet to help beleaguered Assad opponents who have been frustrated by a lack of U.S. assistance after Obama stepped back from launching airstrikes on Syria nearly a year ago.

Senior administration officials have long debated how best to help rebels in Syria's 3-year-old civil war, which is showing signs of spilling over into neighboring Iraq.