WASHINGTON (Reuters) - The U.S. Senate on Thursday was poised to approve President Barack Obama's plan for training and arming moderate Syrian rebels to battle Islamic State militants.

A successful Senate vote on the measure, which passed the House of Representatives on Wednesday on a vote of 273-156, would send it to Obama to sign into law.

Obama plans to use the congressional authority to "degrade and destroy" violent Islamic State militants who have been taking control of large parts of Iraq and Syria - moves that U.S. officials argue threaten America's national security.

The initiative has been attached to a bill that would keep the U.S. government operating on Oct. 1, the start of a new fiscal year. Under the bill, Obama's authority to train and arm moderate rebels would expire on Dec. 11.

The legislation is likely just the start of a debate over what longer-term role the U.S. military should have in battling militants who have seized a third of both Iraq and Syria, declared war on the West and are thought to be responsible for beheading two American journalists in recent weeks.

The U.S. Congress begins a long recess at the end of this week so lawmakers can return to their home states to campaign for re-election in the Nov. 4 midterm elections.

When they come back for a likely short, end-of-year session after the elections, they are expected to return to the question of battling Islamic State militants.

Many Democrats in Congress, including House Democratic leader Nancy Pelosi, have vowed to oppose the use of U.S. combat troops in any expanded military effort the Obama administration might want to conduct. Meanwhile, many Republicans have argued that Obama's plan will fall short of what is needed to stop Islamic State militants.

The United States withdrew its forces from Iraq in 2011 after eight years of conflict.

(Reporting by Richard Cowan and Patricia Zengerle; Editing by John Whitesides and Howard Goller)