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A White House statement said rebels would be vetted before being given assistance, in an effort to assuage concerns that some equipment provided to the Syrian opposition might ultimately fall into the hands of US enemies.
Obama has been under strong pressure from some lawmakers, such as Republican Senator John McCain Of Arizona, to increase assistance to the rebels in Syria's three-year-old civil war. Some lawmakers have accused Obama of being passive and indecisive for months, allowing Assad to repulse a threat to his government.
Obama's request for $500 million followed through on a promise he made in late May in a foreign policy speech that he would "ramp up support for those in the Syrian opposition who offer the best alternative to terrorists and brutal dictators."
The White House said the money would help defend the Syrian people, stabilize areas under opposition control, facilitate the provision of essential services, counter terrorist threats, and promote conditions for a negotiated settlement.
"This funding request would build on the administration's longstanding efforts to empower the moderate Syrian opposition, both civilian and armed, and will enable the Department of Defense to increase our support to vetted elements of the armed opposition," the White House said.