North Korea said it tested a hydrogen bomb on Wednesday, displeasing China, its main ally, and the United States, which said it believed the blast was an ordinary atomic test rather than a much more powerful hydrogen bomb.
In a show of force and support for allies in the region following North Korea's nuclear test, its fourth since 2006, the United States on Sunday sent a nuclear-capable B-52 bomber based in Guam on a flight over South Korea.
North Korea's Rodong Sinmun newspaper, the mouthpiece of the ruling Workers' Party, said the United States was bringing the situation to the brink of war.
South Korean media said the United States may send to South Korea B-2 bombers, nuclear-powered submarines and F-22 stealth fighter jets.
A South Koran defence ministry spokesman declined to give details.
"The United States and South Korea are continuously and closely having discussions on additional deployment of strategic assets," the spokesman, Kim Min-seok, said.
In Washington, the U.S. official said they were discussing deploying "the whole range" of such assets but this meant such things as nuclear-capable bombers rather than restoring U.S. nuclear weapons to South Korea for the first time in about a quarter century.
Former U.S. President George H.W. Bush decided in 1991 to remove U.S. nuclear weapons from South Korea.
"It could quickly escalate into an arms race, a very dangerous arms race, in the region," said the U.S. official, who spoke on condition of anonymity. Asked if such a step might spur North Korea to move more aggressively on its atomic weapons program, the official replied: "That's a distinct possibility."
Putting U.S. nuclear arms back in South Korea, he said, "would embolden the North Korean leadership to be more committed to pursuing their (weapons of mass destruction) capabilities and in fact it would give them a very convenient excuse to do so."
China called for all sides to avoid raising tensions.
"We hope all parties can maintain restraint, proceed cautiously, and avoid successively escalating tensions," said Chinese Foreign Ministry spokesman Hong Lei when asked about the U.S. B-52 flight.
'Highest Level Readiness'
The chairman of South Korea's Joint Chiefs of Staff warned that North Korea was likely to carry out further "sudden provocations", a South Korean defence ministry official said.
The commander of the 28,500 U.S. troops in South Korea, General Curtis Scaparrotti, urged them to be vigilant.
On the diplomatic front, South Korea said its chief nuclear negotiator planned to meet his U.S. and Japanese counterparts on Wednesday to discuss a response to North Korea, and the next day, he would meet China's nuclear envoy in Beijing.
South Korea and Japan used a military hotline for the first time after North Korea's test, South Korea's defence ministry said, in a sign the North is pushing the two old rivals closer together.
South Korea also said it would restrict access to the jointly run Kaesong industrial complex just north of the heavily militarised inter-Korean border to the "minimum necessary level" from Tuesday.
The complex, where South Korean factories employ North Korean workers, is an important source of revenue for the impoverished North.