WASHINGTON, Jan. 20 -- U.S. President Donald Trump signed a legislation Friday to clear a legal barrier for James Mattis, his nominee for defense secretary, to take office.

Trump signed the waiver to grant Mattis a one-time exception, in one of the first actions after he was sworn in as the 45th U.S. President at a ceremony held earlier at the Capitol Hill.

Under the U.S. law, a former service member is barred from running the Pentagon less than seven years after retirement from military. Mattis only retired in 2013 as the Commander of the U.S. Central Command.

The Senate Armed Services Committee voted 24-3 on Jan. 12 to approve Mattis' nomination, moving the vote onto the Senate floor. The committee also easily passed a waiver exempting Mattis from a law that requires defense secretaries to be retired from service for at least seven years.

Mattis' nomination is expected to be approved at a vote in the Senate later Friday.

At the hearing, Mattis took a clear anti-Russia stance during the hearing, describing Moscow as a "principle threat" to U.S. security, a position notably different from Trump.

The United States should also check Russia's growing influence in the Arctic region, Mattis said.

Mattis also chose to stand apart from Trump on other key issues, including the role of NATO, which Trump said should shrink, and on the Iranian nuclear agreement, which Trump threatened to scrap.