President Barack Obama sent Congress legislation on Wednesday to authorize military force against the Islamic State of Iraq and the Levant, asking lawmakers to "show the world we are united in our resolve" to defeat armed fighters who have overrun parts of the Middle East and threaten attacks on the United States.

In urging Congress to back military force, the president ruled out "enduring offensive combat operations," a deliberately ambiguous phrase designed to satisfy lawmakers with widely different views on any role for U.S. ground troops. But the White House said troops may be used on the ground in hostage rescue operations.

Republicans in Congress responded warily to the request.

Speaker John Boehner, R-Ohio, expressed doubt it would "give our military commanders the flexibility and authorities they need to succeed and protect our people." He said changes were likely before the measure comes to a vote. An initial House committee hearing on the authorization was set for Thursday.

Senate Majority Leader Mitch McConnell, R-Ky., like Boehner, said the proposal would receive serious consideration.

There was no timetable for Congress to act on the president's request, which triggers the first war powers vote in Congress since President George W. Bush sought and won an authorization in 2002 before the U.S. invasion of Iraq.

In a letter to lawmakers that accompanied the three-page draft legislation, Obama said ISIL "poses a threat to the people and stability of Iraq, Syria and the broader Middle East and to U.S. national security."

While asking Congress to bar long-term, large scale ground combat operations like those in Afghanistan and Iraq, Obama said he wants the flexibility for ground combat operations "in other more limited circumstances." Those include rescue missions, intelligence collection and the use of special operations forces in possible military action against ISIL leaders.

The issue of ground forces is likely to prove difficult in the administration's attempt to win passage of legislation.

While some Republicans favor their use, many Democrats oppose it, mindful of the long and deadly war in Iraq.

The House Democratic leader, Rep. Nancy Pelosi of California, issued a statement that refrained from endorsing Obama's proposal. It said Congress should act judiciously and promptly to pass legislation "narrowly tailored" to the fight against ISIL. She said previously she opposes deploying U.S. boots on the ground.

Obama arranged to speak publicly about the request later Wednesday.

In his letter, he referred to four American hostages who have died in ISIL custody — at least three of them beheaded. He said the group, if left unchecked, "will pose a threat beyond the Middle East, including to the United States homeland."

Among the four hostages was Kayla Mueller, a 26-year-old humanitarian worker whose death under unknown circumstances was confirmed Tuesday. In addition, the group has urged sympathizers to attacks Western targets.

Obama proposed a three-year limit on the authorization, which would remain in force through the first year of his successor's term in office.

He also proposed no geographic limitations where U.S. forces could pursue the combatants. The authorization covers ISIL and "associated persons or forces," defined as those fighting on behalf of or alongside ISIL "or any closely related successor entity in hostilities against the United States or its coalition partners."

Obama's resolution would repeal a 2002 authorization for force in Iraq but maintain a 2001 authorization against Al-Qaeda in Afghanistan. He said in his letter to lawmakers that his goal is to refine and ultimately repeal that measure as well.

The silence on the 2001 authorization drew criticism from some Democrats. "It makes little sense to place reasonable boundaries on the executive's war powers against ISIL while leaving them unchecked elsewhere," said Rep. Chris Van Hollen, D-Md., in a statement.

At the White House, press secretary Josh Earnest said the ground troop limitation would allow special operations missions, such as potential raids targeting ISIL leaders and the failed attempt last summer to rescue Mueller and other hostages held by the group.

"It's impossible to envision every scenario where ground combat troops might be necessary," he said in an interview with The Associated Press. "The president believes this sort of strikes the right balance of enforcing what he has indicated is our policy while preserving the ability to make some adjustments as necessary."

Obama has said the congressional authorizations that President George W. Bush used to justify military action after the Sept. 11 attacks are sufficient for him to deploy more than 2,700 U.S. troops to train and assist Iraqi security forces and conduct airstrikes against targets in Iraq and Syria.

Obama cast the measure as sending an important message to America's allies and enemies. "I can think of no better way for the Congress to join me in supporting our nation's security than by enacting this legislation, which would show the world we are united in our resolve to counter the threat" from ISIL, he wrote to lawmakers.

Al Jazeera and The Associated Press