Washington, Jan 14: The US Congress has approved a legislation that allows President-elect Donald Trump's pick, retired Marine General James Mattis to head the Pentagon.

The House on Friday easily cleared the bill despite complaints from Democrats that Republicans had rushed the measure unnecessarily.

The bill grants a one-time exception for Mattis from the law that bars former service members who have been out of uniform for less than seven years from holding the top Pentagon job, Fox News reported.

The restriction is meant to preserve civilian control of the military. Mattis, 66, retired from the Marine Corps in 2013.

Congress last granted an exception to the law in 1950 for George Marshall, a former five-star Army general and Secretary of State.

The Senate overwhelmingly passed the legislation on Thursday by an 81-17 vote with 30 Democrats backing the bill.

White House spokesman Josh Earnest said outgoing President Barack Obama would sign the measure if Congress approved it before he leaves office.

Adam Smith of Washington, the top Democrat on the House Armed Services Committee, said the panel held no substantive hearings to examine the impact of the legislation.

He also faulted Republicans for not pushing back when the Trump transition team refused to allow Mattis to testify before the panel on Thursday afternoon after he had completed his confirmation hearing in the Senate.

"We are being treated as irrelevant," Smith said.

Mac Thornberry of Texas, the committee's top Republican, blamed the transition team for "short-sightedness", but rejected Smith's assessment.

"We're not irrelevant because if we do not vote for this legislation, (Mattis) does not serve," Thornberry said.

He also said passing the bill before Trump is sworn in ensures a seamless transition at the Pentagon at a time when the US faces an array of threats.

"The fact is unless we pass this bill today we are not able to have a Secretary of Defense on January 20," Thornberry said. The House will not be in session next week.

During a military career that lasted four decades, Mattis served in numerous senior military positions, including commander of US Central Command in charge of all American forces in the Middle East.

US Congress approves waiver to let Mattis head Pentagon