White House to press Congress for support to fight Islamic State
Fighters from the al-Qaida linked Islamic State of Iraq and the Levant (ISIL) marching in Raqqa, Syria (AFP)

U.S. lawmakers pledged broad support for President Barack Obama's plan to expand the military fight against Islamic State militants in Iraq and Syria, but fears of an open-ended conflict could complicate a vote to back the effort.

Administration officials planned to make their case before Congress on Thursday for a broad escalation of operations against the militant group, including U.S. airstrikes in Syria for the first time and more attacks in Iraq.

The officials will brief all members of Congress and make their case for congressional action supporting the plan Obama laid out Wednesday night in a nationally televised speech.

The White House has asked lawmakers to vote to authorize $500 million in aid for moderate Syrian rebels who are fighting Islamic State on the ground, saying it only needs Congress' approval for training and equipping Syrian rebels.

Administration officials see the vote as crucial to its efforts to build an international coalition in the military fight against Islamic extremists. But the timing is tricky, as such a move could be seen as a "war vote" just two months before congressional elections amid a war-weary public.

A vote could prove especially tough for Democrats, whose liberal base tends to oppose military action, as they try to maintain their slim majority in the U.S. Senate. Many Republicans, who hope to capture control of both houses of Congress in the November election, have long criticized the administration for failing to take action sooner.

One of Obama's harshest critics, Republican Senator John McCain, who has long advocated a stronger U.S. involvement in Syria, criticized Obama as having waited too long to help the opposition groups fighting President Bashar al-Assad.

Asked what one thing the president could tell him to win his support, McCain told CNN, "Airstrikes in Syria begin tomorrow."

Still, recent beheadings of two U.S. captives by Islamic State militants has increased public support for action and coalesced backing from lawmakers on both sides of the aisle.

Before the briefings, some lawmakers on Thursday focused less on the need for a formal "war vote" and more on efforts to turn the tide against Islamic State, including the possibility for expanded role for American troops.

Florida Republican Marco Rubio, a member of Senate Foreign Relations Committee, said he would vote to support action.

"It's important ... to carry this out. I want to see a good plan and certainly want to make sure it's an effective one, not a symbolic one," Rubio told CNBC.

U.S. Representative Adam Smith of Washington state, the ranking Democrat on the House Armed Services Committee, told MSNBC that despite the varying degrees of support from lawmakers so far, "going forward I think there is consensus."

Smith said it was not clear how such a vote would be carried out in Congress. How that measure is defined could matter to some lawmakers.

"This could be taken by some as a war vote," House Appropriations Committee Chairman Hal Rogers said on Wednesday, adding that he has reservations about providing weapons to moderate Syrian rebels that could fall into enemy hands.

(Additional reporting by Doina Chiacu; Editing by Doina Chiacu)