START: Soldiers stand near weapons  displayed last Monday on September 1  2014 at the Makoanyane Barracks in Maseru, during a press conference to present 130 firearms and a number of explosives confiscated from the police after military raided police installations. Picture: AFP/MUJAHID SAFODIEN

SOUTH Korea is in talks with the US to deploy US strategic weapons on the Korean peninsula, a South Korean military official said yesterday, a day after North Korea said it had tested a hydrogen bomb successfully.

South Korea also said it would resume propaganda broadcasts by loudspeaker into North Korea from today, which is likely to infuriate its isolated rival, in response to its fourth nuclear test.

The US and weapons experts voiced doubts that the device tested on Wednesday was a hydrogen bomb.

"The initial analysis is not consistent with the claim the regime has made of a successful hydrogen bomb test," White House spokesman Josh Earnest told reporters.

The underground explosion angered China, which was not given prior notice although it is North Korea’s main ally, pointing to a strain in their ties.

The test also alarmed Japan. Prime Minister Shinzo Abe, agreed with US President Barack Obama in a telephone conversation that a firm global response was needed, the White House said.

Mr Obama also spoke to President Park Geun-hye of South Korea to discuss options.

A South Korean military official said the two countries had discussed the deployment of US strategic assets on the divided Korean peninsula, but declined to give further details.

After North Korea last tested a nuclear device, in 2013, Washington sent a pair of nuclear-capable B-2 stealth bombers on a sortie over South Korea in a show of force. At the time, North Korea responded by threatening a nuclear strike on the US.

South Korea, technically in a state of war against North Korea, said it was not considering a nuclear deterrent of its own, despite calls from ruling party leaders. The US was highly unlikely to restore the tactical nuclear missiles it removed from South Korea in 1991, experts said.

The test was a "grave violation" of an August agreement by the two Koreas to ease tension and improve ties, a South Korean national security official, Cho Tae-yong, said in a statement.

"Our military is at a state of full readiness, and if North Korea wages provocation, there will be firm punishment."

The South raised its military alert to the highest level in areas along the border near its propaganda loudspeakers, the South’s Yonhap news agency reported late yesterday.

The US was limited in its military response for fear of provoking an unpredictable regime in Pyongyang, said Anthony Cordesman, a defence policy expert at the Washington-based Center for Strategic and International Studies think-tank.

"Any escalation in this region, any overreaction can easily lead to not only a conflict between South and North Korea, but drag China and the US and Japan into a confrontation," he said.

Chinese foreign ministry spokeswoman Hua Chunying called for a resumption of talks between the two Koreas, China, the US, Japan and Russia.

Hours after the test, the United Nations Security Council said it would work on new measures against North Korea. Diplomats said that could mean an expansion of sanctions, although major powers might baulk at an all-out economic offensive.