Esper was sworn in at the White House by U.S. Supreme Court Justice Samuel Alito in a ceremony hosted by President Donald Trump and attended by a number of Republican lawmakers. He was confirmed by the U.S. Senate on a vote of 90-8 several hours earlier.

"That's a vote that we're not accustomed to, Mark. I have to say that, so congratulations," Trump told Esper, a former professional staff member of the Senate Foreign Relations Committee and Senate Governmental Affairs Committee.

Esper, 55, a former soldier and lobbyist for weapons maker Raytheon Co , received strong bipartisan support despite sharp questioning during his confirmation hearing by Democratic Senator Elizabeth Warren about his ties to Raytheon and his refusal to extend an ethics commitment he signed in 2017 to avoid decisions involving the company.