The United States' B-52 Stratofortress strategic bomber flew in the skies of South Korea on Sunday in a major show of force four days after North Korea conducted what it said was its first hydrogen bomb test.

The B-52 bomber left U.S. Andersen Air Force Base in Guam on Sunday morning and arrived in the skies above Osan, Gyeonggi Province, at noon, armed with nuclear missiles and "bunker buster" bombs that are capable of bombarding North Korea's underground facilities, according to Seoul and Washington.

The bomber flew past Osan surrounded by an entourage of two South Korean F-15Ks and two U.S. F-16s, the two sides' militaries said.

The deployment was made as part of a series of military counteractions taken after the North announced the nuclear test on Wednesday.

Two days after the test, South Korea resumed its anti-North loudspeaker broadcasts along the inter-Korean border, designed to agitate front-line North Korean soldiers.

The strategic bomber is part of a "nuclear umbrella" the U.S. provides to the Northeast Asian ally.

"The speedier-than-expected deployment could indicate signs of the U.S.' intention that it will retaliate severely if the North makes further provocations," a South Korean military official said.

Officials said the B-52 is capable of bombing North Korea's military commanding facilities from a location some 3,000 kilometers away and it takes only four hours for the U.S. to bring the bomber from the Guam base to the Korean Peninsula.

Officials said South Korea and the U.S. plan to take further military counteractions down the road in the aftermath of the recent nuclear test.  (Yonhap)