The two bombers took off at short notice from Barksdale Air Force Base in Louisiana for the non-stop, 36-hour mission to cross Europe and then the Arabian Peninsula to the Gulf, looping near Qatar while keeping a "safe distance" from Iran's coastline, US defense officials said.

The mission, the second such in two months, "was designed to underscore the US military's commitment to its regional partners, while also validating the ability to rapidly deploy combat power anywhere in the world," according to US Central Command commander General Frank McKenzie.

While McKenzie did not name Iran as the focus of the mission, the US has used shows of force in the air and on the sea to deter Tehran from "malign behavior" toward US forces and allies in the Gulf.

"Potential adversaries should understand that no nation on earth is more ready and capable of rapidly deploying additional combat power in the face of any aggression," said McKenzie.

The flight was coordinated with US allies, and aircraft from Saudi Arabia, Bahrain and Qatar flew with the bombers as they traversed the airspace, according to a US defense official.

In the last year President Donald Trump has ordered the Pentagon to slash US troop numbers in Iraq to just 2,500 by mid-January, the lowest level since 2003.

Likewise, he is cutting the number of troops on the ground in Afghanistan and Somalia.

But the US Navy has kept a significant presence in the region.

In late November a carrier group led by the nuclear-powered USS Nimitz sailed into the Gulf.

"The return of Nimitz is centered on maintaining Centcom's ability to remain postured and prepared to help preserve regional stability and security," US 5th Fleet spokeswoman Commander Rebecca Rebarich said at the time.