US flies supersonic bombers over South Korea after North’s nuclear test

The US sent two nuclear-capable supersonic bombers streaking over its ally South Korea in a show of force aimed at North Korea after its recent nuclear test.

The B-1B bombers, escorted by US and South Korean jets, were seen by an Associated Press photographer as they flew over Osan Air Base, which is 75 miles from the border with North Korea.

The bombers were expected to return to Andersen Air Force Base in Guam, without landing in South Korea.

Such flyovers are common when tension rises on the Korean Peninsula, which is technically in a state of war as there has never been a peace treaty to officially end the 1950-53 Korean War.

South Korea does not have nuclear weapons and relies on the US "nuclear umbrella" as a deterrent to North Korea. Washington also stations more than 28,000 troops in the South, and tens of thousands more in Japan.

North Korea is keenly aware of the US presence on the peninsula and of what it considers the US nuclear threat.

It uses such flyovers and the American military influence in the South in its propaganda as alleged proof of US hostility that it claims as the reason it needs a nuclear bomb programme.

Last week's nuclear test, the North's fifth, was its most powerful to date.

Pyongyang's claim to have used "standardised" warheads in the detonation makes some outsiders worry that it is making headway in its push to develop small, sophisticated warheads that can be mounted on missiles that can reach the US mainland.

International diplomatic efforts to rid the North of its bombs have been stalled since the last round of meetings in late 2008. Since then, Pyongyang has ramped up both its ballistic missile and nuclear bomb development, despite an increasing list of sanctions.

After the test, the North's nuclear weapons institute said it will take unspecified measures to further boost its nuclear capability, which analysts suggested hinted at a possible sixth nuclear test.

South Korea's defence ministry spokesman Moon Sang Gyun said South Korean and US intelligence authorities believe North Korea has the ability to detonate another atomic device at any time at one of its tunnels at its main Punggye-ri nuclear test site, where the five previous atomic explosions took place.

The South's Yonhap news agency reported on Monday that there were signs the North had finished test preparations at one tunnel that has never been used.

Seoul, Washington and their allies have vowed to apply more pressure and sanctions after the test, the second this year.

"The United States and (South Korea) are taking actions every day to strengthen our alliance and respond to North Korea's continued aggressive behaviour," said General Vincent Brooks, commander of US Forces Korea.