REPUBLIKA.CO.ID, 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 was escorted by a South Korean jet and an American jet.

The B52 circled once over the base before heading back to Andersen Air Base in Guam, where it is stationed.

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.

Lieutenant General Terrence J. O'Shaughnessy, commander of the US 7th Air Force and Deputy Commander of the US Forces Korea, said the United States maintains an "ironclad" commitment to the defence of South Korea.

This commitment includes "extended deterrence provided by our conventional forces and our nuclear umbrella", he said in a press statement.

"B52 missions reinforce the US commitment to the security of our allies and partners, and demonstrate one of the many alliance capabilities available for the defence" of South Korea, he said.

"As demonstrated by today's mission, the combined US and Republic of Korea air forces work and train together closely every day, and we are totally prepared to meet any threat to our alliance."

The annual US-South Korea joint military exercises regularly spark angry reactions from North Korea, which brands them "nuclear war drills" against it.

The next such iteration of the Key Resolve/Foal Eagle exercises is expected to take place in March.

South Korea hosts 28,000 US troops as the two Koreas technically remain at war because the Korean War of 1950-53 ended in an armistice instead of a peace treaty.

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