North Korea has confirmed its test of a hydrogen bomb meant for an intercontinental ballistic missile was a 'perfect success'.

The country's sixth nuclear test - 10 times more powerful than its fifth - sparked a powerful 6.3 magnitude earthquake which was detected in the northeast of the country where North Korea's test site Punggye-ri is located.

The hydrogen bomb test ordered by leader Kim Jong-Un was a 'meaningful' step in completing the country's nuclear weapons programme, state television said.

The statement read: 'Scientists in the nuclear field of the DPRK [Democratic People's Republic of Korea] successfully carried out a test of H-bomb for ICBM [intercontinental ballistic missile] in the northern nuclear test ground of the DPRK at 12:00 on September 2, true to the Workers' Party of Korea's plan for building a strategic nuclear force.'

It said radioactive material did not leak into the environment. 

The announcement was delivered by news anchor, Ri Chun-hee - who has been making announcements on Korean Central Television for more than 40 years. 

North Korean television also released photographs appearing to show Kim signing the order to carry out the country's sixth nuclear test. 

The test is a direct challenge to President Donald Trump, who hours earlier had talked by phone with Japanese Prime Minister Shinzo Abe about the 'escalating' nuclear crisis in the region. South Korea's Joint Chiefs of Staff in Seoul said in a statement its military has strengthened monitoring and readiness and is mulling a variety of possible responses that could be pushed together with its ally the U.S.

It says it detected a seismic wave from 12.34pm to 12.36pm Sunday around Punggye-ri.  

South Korea's weather agency and the Joint Chiefs of Staff said an artificial 5.7 magnitude quake occurred at 12.29pm local time, in Kilju, northern Hamgyong province, the site where North Korea has conducted nuclear tests in the past. 

Seoul officials revised their earlier estimate of 5.6 magnitude quake. The U.S. Geological Survey called the first quake an explosion with a magnitude 6.3.

The US State Department had no immediate reaction. South Korea's presidential office said it will hold a National Security Council meeting chaired by President Moon Jae-in.

'It is absolutely unacceptable if North Korea did force another nuclear test, and we must protest strongly,' Japanese Prime Minister Shinzo Abe said.

China's earthquake administration detected a second tremor of magnitude 4.6 in North Korea minutes after the first. It described the event as a cave-in. South Korea's weather agency, however, said no second quake occurred. A US researcher told the BBC that if the earthquake was caused by a nuclear blast, it would be the largest atomic test conducted by North Korea.

Dave Schmerler, of the James Martin Center for Nonproliferation Studies: 'This would be the largest nuclear test that North Korea has ever conducted. We should definitely be alarmed.' 

Japan's Ministry of Defence has dispatched three military jets to test for radiation.  

Tremors caused by the nuclear test were at least ten times as powerful as the last time Pyongyang exploded an atomic bomb a year ago, the Japan Meteorological Agency (JMA) said at a briefing aired by public broadcaster NHK.

The previous nuclear blast in North Korea is estimated by experts to have been around 10 kilotons.

South Korea's defence committee says the blast was about 100 kilotons - powerful enough to destroy an entire city, BBC reports.

People in the Vladivostok in eastern Russia said they felt the tremors.