Neutral Switzerland is prepared to act as a mediator to help resolve the North Korean nuclear crisis, Swiss President Doris Leuthard has said

Ms Leuthard said Swiss troops were deployed on the demarcation zone between South Korea and North Korea and the country had a long history of neutral diplomacy.

But China and the United States had to take their share of responsibility, she added.

"We are ready to offer our role... as a mediator," Ms Leuthard told a news conference.

"It is really time now to sit down at a table. Big powers have a responsibility."

South Korea said it was talking to the United States about deploying aircraft carriers and strategic bombers to the Korean peninsula after signs North Korea might launch more missiles in the wake of its sixth and largest nuclear test.

The UN Security Council has scheduled a second emergency meeting in a week about North Korea after a powerful nuclear test explosion added another layer of urgency for diplomats wrestling with what to do about the North's persistent weapons programmes.

Scheduled after North Korea said it detonated a hydrogen bomb underground on Sunday, the emergency session comes six days after the council strongly condemned Pyongyang's "outrageous" launch of a ballistic missile over Japan.

Less than a month ago, the council imposed its stiffest sanctions so far on the reclusive nation.

North Korea is "deliberately undermining regional peace and stability," the council said when it rebuked the latest missile test, reiterating demands for the country to halt its ballistic missile and nuclear weapons programmes.

Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Show all 6 1 / 6 Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Photos released by North Korea show Kim Jong-un talking to subordinates next to a device thought to be the new thermonuclear weapon. There is no way of independently verifying the pictures STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korea claims it has successfully tested an advanced hydrogen bomb which could be loaded onto an intercontinental ballistic missile AFP/Getty Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A diagram on the wall behind Mr Kim shows a bomb mounted inside a cone STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korean leader Kim Jong-Un (C) attending a photo session with participants of the fourth conference of active secretaries of primary organisations of the youth league of the Korean People's Army (KPA) in Pyongyang STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters

North Korea said it tested an advanced hydrogen bomb for a long-range missile on Sunday, prompting a warning of a "massive" military response from the United States if it or its allies were threatened.

"We are not looking to the total annihilation of a country, namely North Korea," US Defense Secretary Jim Mattis said after meeting Donald Trump and his national security team.

"But as I said, we have many options to do so."