The US navy is concerned about tensions on the Korean Peninsula after a nuclear device was tested in Pyongyang, the capital of North Korea.
A senior US officer said last week that it is a move that could prompt tighter military ties between neighbours Japan and South Korea.
“A huge explosion occurred, and that should make all countries concerned,” Vice Admiral Joseph Aucoin, commander of the US Seventh Fleet, told Reuters reporters.
He said the Seventh Fleet stood ready to support US forces in South Korea if necessary.
A recent landmark agreement seeking to end tensions over Korean women and girls forced to work in wartime Japanese brothels has improved relations between Japan and South Korea.
Doubts have been voiced by the US government and weapons experts about whether the device tested by North Korea on Wednesday was a hydrogen bomb, as Pyongyang had claimed.
The British foreign secretary, Philip Hammond, has urged South Korea to ‘exercise restraint’ in its response to North Korea’s nuclear test, after Seoul resumed propaganda filled broadcasts across the border that the regime in Pyongyang regards as an act of war.
Speaking during a visit to Japan on Friday, Hammond said he understood why South Korea had retaliated by restarting the broadcasts to coincide with what is thought to be the 33rd birthday of the North Korean leader, Kim Jong-un.
Hammond told reporters: “North Korea acts in a totally provocative and irresponsible way, so I can understand the pressure South Korea is under to respond.”
But he added: “We have to be bigger than the North Koreans and urge South Korea and other like-minded countries in the region to exercise restraint.”
Josh Earnest, spokesman for President Barack Obama, has disputed North Korea’s claim that it had detonated a Hydrogen bomb.
He said: “The initial analysis that has been conducted… is not consistent with North Korean claims of a successful Hydrogen bomb test.”
Mr Earnest says the test does not lead the US to believe North Korea has expanded its military capability.