* North Korea nuclear test stokes world condemnation

* Obama speaks to Japan, South Korea leaders

* Doubts over whether nuclear test was a hydrogen bomb

* China angered by ally North Korea

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

The US and weapons experts voiced doubts the device was as advanced as North Korea said, but calls mounted for more sanctions against the isolated state for its rogue nuclear programme.

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

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

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

A South Korean military official told Reuters 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 the North, said it was not considering a nuclear deterrent of its own, despite calls from ruling party leaders. The US is highly unlikely to restore the tactical nuclear missiles it removed from South Korea in 1991, experts said.

Military response

The US is also 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 over-reaction can easily lead to not only a conflict between South and North Korea, but drag China and the US and Japan into a confrontation," Cordesman said.

A Chinese foreign ministry spokeswoman called for a resumption of so-called six-party talks between the two Koreas, China, the US, Japan and Russia aimed at curbing North Korea's nuclear ambitions.

"We are worried about how things are developing," the spokeswoman, Hua Chunying told a briefing when asked if US weapons to South Korea risked inflaming the situation.

Asked about a suggestion from US Republican presidential front-runner Donald Trump that China could do more to rein in North Korea, Hua said: "What constructive efforts have they made?"

Hours after the nuclear test on Wednesday, the UN Security Council said it would work immediately on significant 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.

US congressional sources said Republican leaders of the US House of Representatives were considering a vote as soon as next week to impose stiffer punishment on foreign companies doing business with North Korea.

Bellicose rhetoric   

North Korea has a long history of bellicose rhetoric against the US and its Asian allies but its assertion that it had tested a hydrogen device, much more powerful than an atomic bomb, came as a surprise.

North Korea also said it was capable of miniaturising the H-bomb, in theory allowing it to be placed on a missile and threatening the US West Coast, South Korea and Japan.

The US State Department confirmed North Korea had conducted a nuclear test but the Obama administration disputed the hydrogen bomb claim.

"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 test took place two days ahead of what is believed to be North Korean leader Kim Jong Un's birthday.

North Korea called the device the "H-bomb of justice", but its state news agency also said it would act as a responsible nuclear state and would not use its nuclear weapons unless its sovereignty was infringed.

The impoverished state boasts of its military might to project strength globally but also plays up the need to defend itself from external threats as a way to maintain control domestically, analysts say.

Hydrogen bombs use a two-step process of fission and fusion that releases substantially more energy than an atomic bomb. However, it will likely take several days to determine more precisely what kind of device was set off as a variety of sensors, including "sniffer planes", collect evidence.

A US government source said Washington believes North Korea had set off the latest in a series of tests of old-fashioned atomic bombs.

The size of the latest explosion was roughly consistent with previous tests and occurred in the same location.

The US had been anticipating a North Korean nuclear test for some time, with intelligence indicating possible preparations such as evidence of new excavations of underground tunnels at the site.

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