U.S. lawmakers on Sunday stepped up calls for congressional authorization of President Barack Obama's war against Islamic State militants in Iraq and Syria, amid signs the United Statesand its allies face a long and difficult fight.

U.S. House Speaker John Boehner told ABC's "This Week" that he believed Obama had the legal authority for strikes against Islamic State, but would call lawmakers back from their districts if Obama sought a resolution backing the action.

"I think he does have the authority to do it. But ... this is a proposal the Congress ought to consider," Boehner said, warning that the United States could eventually be dragged into another ground war in the region.

Obama and other U.S. officials have said they believe no further vote to authorize force is needed, but political analysts warn that the war could dampen participation by anti-war Democrats in the November elections.

Senator Chris Murphy, a Connecticut Democrat and member of the Senate Foreign Relations Committee, told CNN that Congress should debate the issue because of uncertainty about how long the U.S. military would remain engaged in Syria.

"There are some serious questions that we have to ask," Murphy said. "You need a realistic political strategy. And I just don't think we have that today in Syria."

Senator John Barrasso, a Wyoming Republican and member of the Foreign Relations Committee, urged Obama to follow the lead of British Prime Minister David Cameron in recalling the British parliament. "I think the president has an obligation to call us back tomorrow to start this debate," Barrasso said.

Tony Blinken, deputy national security adviser, told "Fox News Sunday" that U.S. strikes against Islamic State militants were showing progress, but it would take time to strengthen opposition forces on the ground. "This is going to be a long-term effort," he said.

Blinken said U.S. military action in Syria and Iraq was authorized under a measure passed in 2001 before the first U.S. strikes against Afghanistan. But he said Obama would welcome a more "targeted, focused authorization."

Senator Tim Kaine, a Virginia Democrat, said he backed the administration's strategy for fighting Islamic State, but did not believe that current law and the existing war resolutions gave Obama the authority to carry out the full mission.

"We're not supposed to start a war without Congress," Kaine told CBS's "Face the Nation" program. "It really concerns me that the president would assert he has the ability to do this unilaterally, when as a candidate he made very plain that the president cannot start a war without Congress."

Senator Angus King, an Independent from Maine, said U.S. lawmakers had been ceding their authority to declare war since 1938 when then-President Harry Truman sent the U.S. military into Korea without a formal declaration of war.

"It's time for Congress to stand up, put itself on record and make some decisions here," he said.