WASHINGTON • United States Defence Secretary James Mattis has nominated four-star navy Admiral Phil Davidson to become the top military officer in the Pacific, a move that will keep US Pacific Command (Pacom) under navy leadership.

Adm Davidson's top rival for the Pacific Command job, Air Force General Terrence O'Shaughnessy, has been nominated to lead US Northern Command, relieving General Lori Robinson, reported the Defence News website.

Adm Davidson is a career surface warfare officer and is currently the head of US Fleet Forces Command.

He is a 1982 Naval Academy graduate, and will assume his new role at a time when the US is involved in a nuclear stand-off with North Korea and is contesting China's expansive territorial claims in the South China Sea.

Adm Davidson became the leading navy candidate to oversee Pacom after outgoing US Pacific Fleet commander Admiral Scott Swift abruptly announced his intent to retire on social media in September in the wake of two deadly collisions of US vessels that left 17 sailors dead.

The accidents are understood to have scuttled his prospects for the position, reported the US Naval Intelligence News website.

Gen O'Shaughnessy, on the other hand, was ruled out of a Pacom role after the Pentagon expressed concern that an Air Force officer would not be able to clear a Senate Armed Services Committee led by Senator John McCain, whose father served as Pacom commander from 1968 to 1972.

The outgoing Pacom chief, Admiral Harry Harris, had a reputation for being aggressive with forces deployed in the region.

In November, the US staged a major show of force in which three aircraft carriers conducted a simultaneous patrol in waters off the Korean peninsula.

Adm Harris has been nominated to be the Trump administration's ambassador to Australia.

Adm Davis has spent most of his career on the East Coast and most recently led the navy's Comprehensive Review of readiness issues in the Japan-based 7th Fleet in the wake of the deadly vessel collisions.

He has previously commanded the Eisenhower carrier strike group, the cruiser Gettysburg and the frigate Taylor. Prior to his appointment as Fleet Forces commander, he oversaw the US 6th Fleet in Europe.

According to his official biography, he gained experience early in his career at a staff job in the US Pacific Fleet.

The current commander of the US 6th Fleet, Vice-admiral Chris Grady, was nominated last month to lead Fleet Forces as Adm Davidson's replacement.