WASHINGTON — In a challenge to President Barack Obama's handling of the conflict in Libya, a group of US senators accused him Wednesday of violating a 1973 law aimed at curtailing the White House's war powers.

Conservative Republicans Rand Paul, Jim DeMint, Mike Lee, Ron Johnson, Tom Coburn, and John Cornyn wrote a letter to Obama charging that he had committed US forces "without regard to, or compliance with" the War Powers Act.

The senators pressed Obama to say whether he would abide by a requirement in the law -- ignored by several US presidents -- that US forces start withdrawing from a conflict within 60 days unless explicitly authorized by the Congress.

"Friday is the final day of the statutory sixty day period for you to terminate the use of the United States Armed Forces in Libya under the War Powers Resolution," the lawmakers said in their letter.

"Last week some in your administration indicated use of the United States Armed Forces will continue indefinitely, while others said you would act in a manner consistent with the War Powers Resolution," they said.

"Therefore, we are writing to ask whether you intend to comply with the requirements of the War Powers Resolution. We await your response."

The US Constitution reserves to Congress the right to declare war, though US presidents have often sent forces into combat without first getting lawmakers' explicit say-so, despite the War Powers Act.

The law allows the president to use force in response to an attack on the United States, its territories, or its armed forces, but calls for notifying Congress within 48 hours and says US troops must start to withdraw 60 days later unless specifically authorized to remain by lawmakers.

The act says such a withdrawal can last 30 days.

Earlier, US Defense Secretary Robert Gates sidestepped a reporter's question on whether the administration would ignore the law or whether it would affect US operations in support of the NATO-led, UN-backed mission.

"The War Powers Act question is above my pay grade, and so I would refer you to the White House," he said. "There are many lawyers advising the White House, I am convinced, and I'm not one of them."