A nuclear-weapons-capable B-52 bomber landed in South Korea on Tuesday, Yonhap news agency reported, in the latest show of United States support for long-time ally Seoul in the face of North Korean military threats.

A photo published by Yonhap shows the aircraft landing at Cheongju Air Base, about 110 kilometers south of the capital, after it first made a flyover at South Korea's largest defense exhibition.

While B-52s have previously taken part in joint exercises over the peninsula, Tuesday's development marks the first time one has landed in the country since at least 2000, when such record-keeping began.

The bomber's arrival comes less than a week after a South Korean port visit by the nuclear-powered aircraft carrier USS Ronald Reagan.

The carrier's docking in Busan provoked an angry response from Pyongyang, which said it showed Washington had undertaken the "most serious phase" of preparations for a potential nuclear war.

The US military, which stations around 28,000 soldiers in the South and is treaty-bound to defend it against North Korean attack, declined to confirm Tuesday's bomber landing.

B-52s, which fly at subsonic speeds, can travel more than 8,800 miles without refuelling at an altitude reaching 50,000 feet.

Pyongyang baulks at Washington's deployment of such long-range military assets and in 2017 threatened to "shoot down" US strategic bombers even outside North Korean airspace.

The B-52's arrival comes a day ahead of a planned two-day visit to Pyongyang by Russian Foreign Minister Sergei Lavrov.

Russia and North Korea, historic allies, are both under rafts of global sanctions — Moscow for its invasion of Ukraine, Pyongyang for its nuclear weapon tests.

Moscow is believed to be interested in buying North Korean ammunition, while Pyongyang wants Russia's help to develop its internationally condemned missile program.

On Saturday, the United States said that arms shipments to Russia were already under way, with North Korea delivering more than 1,000 containers of military equipment and munitions in recent weeks.

The Kremlin said on Tuesday that Western allegations that North Korea was supplying weapons to Russia were not based on evidence, the TASS news agency cited Kremlin spokesperson Dmitry Peskov as saying, according to Reuters.