Former soldier Mark Esper was sworn in as US secretary of defense Tuesday after earning Senate confirmation, filling America's longest-ever Pentagon leadership vacuum as Washington faces mounting tensions with Iran and struggles to end the long-running Afghanistan war.

President Donald Trump's second Defense Department chief takes over nearly seven months after the shock departure of Jim Mattis, the deeply respected career US Marine who broke with Trump over policy on the Middle East and Afghanistan.

Two others were made acting defense secretary this year to fill the void - including Patrick Shanahan, who served a six-month temporary stint but resigned for family reasons in June and withdrew from consideration for the full-time top post.

Esper sailed through the confirmation process at lightning speed. He earned broad bipartisan support and was confirmed by a vote of 90 to 8. Later Tuesday, he was sworn in at an Oval Office ceremony attended by Senate Republicans and Trump, who called it "a very important day" for the nation. 

"There is no one more qualified to lead the Department of Defense," the president said.

Esper's confirmation brings ballast to a Pentagon destabilized by the leadership revolving door since late December, and comes as the world's primary military power is engaged in conflicts in countries including Afghanistan and is being tested by Tehran.

Senate Majority Leader Mitch McConnell, the top Republican in Congress, hailed Esper, who served a two-year stint as secretary of the US Army beginning in 2017, as "a thoroughly well prepared nominee" who has the respect of the national security community and can hit the ground running.

"The world is full of serious threats to America, to our allies and to our interests, not least among them obviously is Iran's insistence on continuing to ratchet up tensions in the Middle East," McConnell said in a Senate floor speech Monday. "Having a Senate-confirmed secretary of defense, especially one of this quality, could not come a moment too soon."