South Korea is in talks with the United States to deploy U.S. 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.
South Korea also said it would resume propaganda broadcasts by loudspeaker into North Korea from Friday, which is likely to infuriate its isolated rival, in response to its fourth nuclear test.
The United States and weapons experts voiced doubts the device North Korea tested on Wednesday was a hydrogen bomb, but calls mounted for more sanctions against it 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 their ties.
The test also alarmed Japan. Its prime minister, Shinzo Abe, agreed with U.S. 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 U.S. 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 United States.
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 United States is 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 on Thursday.
The United States is 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 United States 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 United States, 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.
Asked about a suggestion from U.S. 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, the U.N. 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.
SURPRISE
North Korea has a long history of bellicose rhetoric against the United States 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 U.S. West Coast, South Korea and Japan.
The U.S. 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 U.S. government source said Washington believes North Korea had set off the latest in a series of tests of atomic bombs.