President Barack Obama is poised in coming days to ask Congress for new authority to use U.S. military force against Islamic State militants, the White House said Thursday. But the top Republican in Congress warned it won't be easy to pass the measure and that it will be up to Obama 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," House Speaker John Boehner, R-Ohio, said Thursday. "This is not going to be an easy lift."

White House spokesman Josh Earnest responded that the administration is dedicated to getting a new authorization with support from Republicans and Democrats. That's even though Obama has argued new authorization isn't legally necessary and has been ordering airstrikes on militant strongholds in Iraq and Syria for months.

"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," Earnest said, using an acronym for the Islamic State group, "That across branches of government and even across political parties, even in this divided time in our nation's political history, at least, that Democrats and Republicans are committed to this very important task."

Earnest declined to discuss specific provisions being discussed, such as how long the authorization will last, what geographical areas it will cover and whether it will allow for the possibility of ground troops. He said details are still being worked out with lawmakers from both parties, with the hopes of coming up with the authorizations can draw bipartisan support.

But top House Democrat Nancy Pelosi of California said talks with the administration are focusing on a time frame of three years, while the other issues are still being worked out. Pelosi told journalists it will be a challenge for wary Democrats, the White House, and Republicans seeking a broader use of military force to forge an agreement, but that she ultimately expects one to be reached.