By Steve Holland and Patricia Zengerle

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

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

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

A White House statement said Syrian rebels would be "appropriately vetted" before being given assistance, in what amounts to an effort to assuage concerns that some equipment provided to the Syrian opposition might ultimately fall into the hands of U.S. enemies.

Obama has been under strong pressure from some lawmakers, such as Republican Senator John McCain Of Arizona, to increase assistance to the Syrian rebels. Some members of Congress have accused Obama of being passive and indecisive, allowing Assad to repulse the 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.

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Democrats in Congress said they were pleased Obama was taking a step to back the Syrian rebels. "It is not too late to help the moderate opposition. It is not too late to help Syrians build the future they deserve," said U.S. Representative Eliot Engel, the top Democrat on the House of Representatives Foreign Affairs Committee.

Republican Senator Marco Rubio of Florida criticized Obama's "rudderless" foreign policy.

“The consequences of the Obama Administration’s inaction and inability to articulate and implement a coherent Syria strategy is now painfully evident across the Middle East, especially in Iraq, and now threatens U.S. partners such as Jordan," Rubio said in a statement.

Syrian rebels fighting Assad are a disparate array of groups including the radical Islamic State of Iraq and the Levant, which seized parts of northern and western Iraq in an offensive this month; Nusra Front, which is affiliated to al Qaeda; and moderate, non-ideological factions such as the Supreme Military Command.

(Reporting by Steve Holland and Mark Felsenthal; Editing by Sandra Maler and Mohammad Zargham)