An American stealth bomber capable of launching nuclear attacks on anywhere in the world has taken to the skies.

The B-21 Raider, which is worth almost $700m (£516m), has been pictured for the first time.

The images of air-to-air refuelling tests are a rare glimpse of an aircraft still undergoing trials.

Expected to enter service next year, the B-21 will be the world’s most advanced stealth bomber and will take over from the B-2, which has been in service for almost 30 years.

Photographs released by the US air force show the B-21 Raider soaring over mountains in an undisclosed location followed by a KC-135 Stratotanker.

Aerial in-flight refuelling is critical in a potential conflict, enabling the fleet to dramatically extend its reach, as has been shown with US air strikes on Iran.

In order to refuel mid-air, a tanker will fly ahead of an aircraft while deploying a fuel boom behind it. The jet in need of replenishing then approaches with a probe, connecting it to the hose. All this is carried out at hundreds of miles per hour.

The B-21, manufactured by Virginia-based Northrop Grumman, was not refuelled during the test, with the bomber instead working with the tanker as part of a “close proximity flight”.

America’s new stealth bomber consumes a fraction of the fuel used by its predecessor, the B-2 Spirit, making it the “most fuel-efficient bomber ever built”, the US military said.

“This long-range strike bomber will reduce the demand on our tanker fleet and free up assets to support the joint force,” said Gen Ken Wilsbach, the US air force chief of staff.

“This will provide a wider range of employment options and the deterrence our nation requires.

“This capability ensures we can deliver penetrating long-range strikes anywhere in the world, at any time,” added Gen Stephen L Davis, head of the US air force global strike command.

“For our bomber crews and the combatant commanders they support, this is about endurance and mission readiness.”