Washington: Republican and Democratic leaders in Congress will face some resistance in a vote to authorize President Barack Obama's war against Islamic State militants despite international outrage over video of militants beheading their captives and burning one alive.

War authorizations are among the most difficult issues confronting members of Congress. Several Democrats will be reluctant to approve new war powers unless there is a clear deadline or some way to pay for the military operation. Some Republicans, strong foes of the president, will object to giving Obama the authority.

Obama is poised in coming days to ask Congress for new authority to use U.S. military force against IS, the White House said Thursday. But the top Republican in Congress warned it won't be easy to pass the measure. Republican House Speaker John Boehner said it will be up to the president to rally support from lawmakers and the public.

"His actions are going to be an important part of trying for us to get the votes to actually pass an authorization," Boehner said Thursday. "This is not going to be an easy lift."

In the U.S. battle against IS, Obama has been relying on congressional authorizations that President George W. Bush used to justify military action after September 11, 2001 attacks. Critics say the White House's use of post-9/11 congressional authorizations is a legal stretch, at best.

Obama has insisted that he had the legal authority to send U.S. troops to train and assist Iraqi security forces, and to launch airstrikes since September against targets in Iraq and Syria. Now, the administration wants to get a new so-called Authorization for the Use of Military Force, or AUMF, with bipartisan support from Congress.

"The president believes it sends a very powerful signal to the American people, to our allies, and even to our enemies, that the United States of America is united behind this strategy to degrade and ultimately destroy ISIL," White House spokesman Josh Earnest said, using an acronym for the Islamic State group.

Republicans generally want a broader authorization of military action against the militants than Democrats have been willing to consider. Obama has said he does not intend to deploy U.S. combat troops, though many Republicans believe that option ought to be available.

Earnest declined to discuss specific provisions being considered, such as how long the authorization will last, what geographical areas it will cover and whether it will allow for ground troops. He said details were still being worked out with lawmakers from both parties.

Top House Democrat Nancy Pelosi of California said talks with the administration are focusing on an authorization that would last three years, with other issues still being debated. Pelosi told journalists it will be a challenge for Democrats, the White House and Republicans to forge an agreement, but that she ultimately expects one to be reached.

"I'm not saying anybody's come to an agreement on it," Pelosi said. "I think it's going to be a challenge, but we will have it."

Pelosi said she hopes Congress will repeal the 2002 congressional authorization for the war in Iraq but retain the 2001 authorization for military action in Afghanistan. Earnest said the White House also supports repeal of the Iraq authorization replaced by the new authorization.

Late last year, Secretary of State John Kerry said whatever new authorization Congress passes should not limit U.S. military action to Iraq and Syria or prevent the president from deploying ground troops if he later deems them necessary. He also said that if the new authorization had a time limit, there should be a provision for it to be renewed.

 Islamic State militants released a grisly video this week of a Jordanian Air Force pilot being burned alive inside a cage. Pelosi said that the U.S. should "move quickly" to steer military aid to Jordan, which has stepped up a campaign against the militants, including a series of airstrikes in Syria.