Seoul: A US long-range heavy bomber flew over South Korea on Sunday, the US military said, days after North Korea conducted its first alleged hydrogen bomb test in defiance of international sanctions.

The B52 Stratofortress, which is capable of carrying nuclear weapons, briefly roared over the Osan Air Base, some 72 kilometres (45 miles) south of the inter-Korean border, the military and an eye-witness said.

It circled once over the airbase before heading home.

The aircraft are known to have taken part in joint annual US-South Korea military exercises that have enraged Pyongyang, but their flights over South Korea are rarely publicised.

The last time such a flight was made public was in 2013, after North Korea carried out its third nuclear test.

At that time, the US dispatched both a B52 and the more sophisticated B2 stealth bomber to South Korea in a show of military muscle against the North.

Pyongyang on Wednesday conducted its fourth nuclear test, angering the international community and raising tensions across the inter-Korean border.