Syria crisis: Obama asks Congress for $500m for rebels
The funds would help Syrians "defend themselves against regime attacks", the White House said in a statement.
It would also be aimed at countering extremists such as the Islamic State in Iraq and the Levant (Isis), it added.
The advance of Isis in neighbouring Iraq has led to pressure on Mr Obama from some lawmakers.
'Increase our support'
"This funding request would build on the administration's longstanding efforts to empower the moderate Syrian opposition, both civilian and armed," the White House said.
It will also "enable the Department of Defense to increase our support to vetted elements of the armed opposition".
The money will help stabilise areas under opposition control and counter terrorist threats, the White House said.
The rebels that would receive the funds will be vetted beforehand in order to alleviate concerns of equipment falling into US enemy hands, the White House said.
Mr Obama has been under strong pressure from some members of Congress to increase assistance in the area.
Last month Mr Obama hinted at increased help for the Syrian opposition in a speech at the military academy at West Point.
He said he would work with Congress to "ramp up support for those in the Syrian opposition who offer the best alternative to terrorists and a brutal dictator".
BBC