America's defence secretary has warned North Korea that any threat to the US and its allies will be met by a "massive military response".

Following Pyongyang's claims of testing a hydrogen bomb, Jim Mattis warned America's response would be "both effective and overwhelming" if it was put in danger.

Speaking outside the White House, he added: "We are not looking to the total annihilation of a country, namely North Korea. But we have many options to do so."

On Sunday, Kim Jong Un's regime said North Korea's sixth nuclear test was a "perfect success" and involved a bomb designed to be mounted on its newly developed intercontinental ballistic missile.

Despite tough talk from the US, the immediate response from the international community is likely to focus on enforcing even tougher economic sanctions on the isolated state.

According to diplomats, the United Nations Security Council could consider banning North Korea's national airline and textile exports from Pyongyang, as well as halting the supply of oil to its government and military.

North Korean nationals could also be prevented from working abroad, and its top officials could be subjected to an asset freeze.

Such economic penalties have had little effect on Pyongyang's nuclear ambitions in the past.

The UN Security Council has scheduled an emergency meeting - its second urgent session in less than a week.

Meanwhile, South Korea's air force and army have conducted missile drills involving long-range ballistic and air-to-surface missiles.

According to Seoul's joint chiefs of staff, more drills are being prepared with US forces.

The device tested by North Korea on Sunday is thought to have been about five times larger than the bomb dropped on the Japanese city of Nagasaki in World War II.

It even caused a manmade earthquake, with Japanese and South Korean officials claiming that the tremor was up to 10 times more powerful than the one picked up after Pyongyang's previous nuclear test a year ago.

There has been no independent confirmation that the detonation was a hydrogen bomb rather than a less powerful atomic weapon similar to those Pyongyang has tested in the past.