President Barack Obama recently told Congressional leaders that he doesn’t need lawmakers’ approval for any action in Iraq, the top Senate Republican says.
Senator Mitch McConnell was speaking after a meeting between the president and senior members of Congress.
Iraq has asked for US air strikes against advancing jihadist militants, who have seized key cities and towns.
But correspondents say any decision on military support from Washington could hinge on political changes in Iraq.
US Vice-President Joe Biden discussed possible “additional measures” that could help “roll back the terrorists’ advances” with Iraqi Prime Minister Nouri Maliki on Wednesday, but also the need for national unity.
The Obama administration has shown signs of frustration with Mr Maliki – a Shia Muslim who has long been accused of discriminating against the Sunni Arab minority community and monopolising power.
Defence Secretary Chuck Hagel told a congressional hearing: “This current government in Iraq has never fulfilled the commitments it made to bring a unity government together with the Sunnis, the Kurds, and the Shia.”
Mr Obama met Congressional leaders at the White House on Wednesday to discuss the US response to recent advances by the Islamic State in Iraq and the Levant (ISIS).
Speaking afterwards, Mr McConnell said the president had “indicated he didn’t feel he had any need for authority from us for steps that he might take”.
Correspondents say the White House has so far avoided the thorny question as to whether it needs Congressional authority for any military action in Iraq.
Experts say Mr Obama has several options, including citing the Authorisation for the Use of Military Force (AUMF) against al-Qaeda and its associated forces passed by Congress in 2001; the 2002 AUMF that led to the invasion of Iraq; and his own powers as commander-in-chief.
Last year, the president abandoned plans for punitive military strikes in Syria following a deadly chemical weapons attack once it became clear that Congress would not give its backing.
Administration officials say the president may be able to act unilaterally in Iraq because its government has requested US air strikes against ISIS, which seized the second city of Mosul last week and is advancing southwards towards Baghdad.
ISIS and their Sunni Arab allies are reported to be advancing in Diyala and Salahuddin provinces after they overran Iraq’sÂ second city, Mosul, last week.