US President Barack Obama had told Congressional leaders he does not need lawmakers' approval for any action in Iraq, the top Senate Republican says.
Senator Mitch Mc McConnell was speaking after a meeting between the president and senior members of Congress.
Iraq has asked for US air strikes against advancing Sunni militants.
Meanwhile US Vice-President Biden and Iraqi PM Nouri Maliki discussed possible "additional measures" by the US to assist Iraqi forces.
The two men considered ways "to roll back the terrorists' advances", a White House statement said.
On Wednesday Obama met Congressional leaders at the White House to discuss the US response to recent advances by ISIS (Islamic State in Iraq and the Levant).
— Nicholas Kristof (@NickKristof) June 19, 2014
Speaking afterwards, 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.
Last year the president did not seek consent for possible attacks against Syria, although he abandoned such a move once it became clear that Congress would not support it.
Earlier this month a number of lawmakers condemned the lack of congressional consultation over the release of army Sergeant Bowe Bergdahl from the Taliban in Afghanistan in exchange for five Guantanamo Bay detainees.
— The New York Times (@nytimes) June 19, 2014
Administration officials say the president may be able to act unilaterally in Iraq because its government has requested US air strikes against ISIS militants who have seized several key cities over the past week.
ISIS and their Sunni Muslim allies are also reported to be advancing in Diyala and Salahuddin provinces after they overran Iraq's second city, Mosul, last week.
They have also launched an assault on Iraq's biggest oil refinery at Baiji, north of Baghdad.
The administration has not officially responded to Iraq's request for air support in its response to the offensive.
BBC security correspondent Frank Gardner says Obama has a wide range of options, from air strikes to providing extra training, but that the US will - as a minimum - send in surveillance drones.
— Bloomberg News (@BloombergNews) June 18, 2014
Ahead of Wednesday's meeting, Senate leader Harry Reid, a Democrat, said he did not "support in any way" getting American troops involved in the Iraqi "civil war".
The administration itself has shown signs of frustration with Maliki - who has long been accused of favouring the country's Shia Muslim majority and alienating Sunnis.
On Wednesday 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."
Photo op of Pres Obama & Cong Ldrs at start of their Oval Office talks on Iraq. (Frame grab from TV Pool) pic.twitter.com/l9Vy8hEOMp
— Mark Knoller (@markknoller) June 18, 2014
'DANGER TO UNITY'
Hundreds of people have been killed since the start of the militant offensive, many of them believed to be captured soldiers publicly shot by ISIS-led firing squads.
Some 500,000 people have been internally displaced, according to UN estimates.
In an interview with the BBC, the representative of Grand Ayatollah Ali al-Sistani, the country's Shia spiritual leader, said warned against "a real danger which threatens Iraq and its unity".
Speaking to BBC world affairs editor John Simpson, Sheikh Abdul Mahdi Karbalai said the response should be "a stance which is adopted by people from all sects".
Iranian President Hassan Rouhani said Tehran would not "spare any effort" to defend Shia holy shrines in Iraq against "mercenaries, murderers and terrorists".