Kirk Spitzer

USA TODAY

TOKYO – The U.S. military responded to North Korea's claimed test of a hydrogen bomb on Sunday with a menacing demonstration of its own: a low-level flight by a B-52 bomber over neighboring South Korea.

The long-range bomber made a low pass near Osan Air Base, about 40 miles south of Seoul, at about noon local time. It was accompanied by U.S. and South Korean fighter planes, according to a statement from U.S. Pacific Command in Hawaii.

The bomber flew from its home base in Guam.

"This was a demonstration of the ironclad U.S. commitment to our allies in South Korea, in Japan, and to the defense of the American homeland," Adm. Harry Harris Jr., commander U.S. Pacific Command, said in the statement released Sunday.

"North Korea's nuclear test (last Wednesday) is a blatant violation of its international obligations. U.S. joint military forces in the Indo-Asia-Pacific will continue to work with all of our regional allies and partners to maintain stability and security," Harris said.

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Pacific Command took the unusual step of releasing a photo of the B-52 and escort fighters during the flight.

White House chief of staff Denis McDonough said on CNN’s State of the Union that the flight demonstrated “the deep and enduring alliance" the United States has with South Korea and other allies. He said the North Koreans need to follow prior commitments to get rid of their nuclear weapons.

“That’s the baseline requirement they have to rejoin the international community,” McDonough said. “Until they do it, they’ll remain where they are, which is an outcast — unable to provide for their own people.”

There was no immediate response from North Korea about the B-52 flight. Last week's test at Punggye-ri was the country’s fourth test of nuclear weapon and the first in three years.

North Korea announced that the device was a powerful hydrogen bomb, which is far more destructive than the atomic bombs it previously tested. However, the White House challenged that claim, saying initial evidence suggested the detonation was not powerful enough to be a hydrogen bomb.

North Korea’s nuclear weapons and ballistic missile programs are in violation of international arms control agreements.

Sunday's flight is unlikely to deter North Korea from continuing its weapons programs but was a necessary step, said Grant Newsham, senior research fellow at the Japan Forum for Strategic Studies in Tokyo.

“These sorts of concrete responses do keep alive in the North Korea’s mind that the United States — and what it might do — are still worth worrying about. As importantly, it also reassures South Korea and Japan and others that America still can be counted on to defend against North Korean misbehavior," Newsham said.

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B-52 bombers have been the Pentagon’s weapon of choice when looking to send a message in recent years. A B-52 made a similar over-flight following North Korea’s nuclear test in 2013, and the Pentagon sent two of the aircraft into a disputed air defense zone declared by China in the East China Sea in 2013.

It is not clear whether the B-52 that flew Sunday was armed. The warplane has been the backbone of the U.S. bomber fleet since the 1950s and is capable of carrying both nuclear and conventional bombs, and cruise missiles. The plane has been continually upgraded with new engines, electronics and weaponry.

Last week’s nuclear test has worsened relations between North and South Korea, which are still technically at war since 1950. The South resumed loudspeaker broadcasts across the demilitarized zone separating the two countries — an action the North calls an "act of war" — and North Korea moved additional troops into the border area.

In the statement released Sunday, Pacific Command said the B-52 mission was intended to demonstrate the strength of the alliance between the United States and South Korea and the resolve of both nations to maintain stability and security on the Korean Peninsula.