A small tremor detected near North Korea's nuclear test site is probably not manmade, the nuclear proliferation watchdog and a South Korean official said, easing fears Pyongyang had exploded another nuclear bomb just weeks after its last one.

Key points: Quake detected near known nuclear site, South Korea weather agency says

Quake detected near known nuclear site, South Korea weather agency says Nuclear watchdog says seismic events "unlikely [to be] man-made"

Nuclear watchdog says seismic events "unlikely [to be] man-made" Previous quakes from North Korea have indicated nuclear tests

Chinese earthquake officials said the magnitude-3.4 quake detected about 6:30pm AEST on Saturday was a "suspected explosion" but both the CTBTO, which monitors nuclear tests, and a South Korean meteorological agency official said they believed it was a natural quake.

"A key method is to look at the seismic waves or seismic acoustic waves and the latter can be detected in the case of a manmade earthquake," said a South Korean official, who asked for anonymity.

"In this case we saw none. So as of now, we are categorising this as a natural earthquake."

The earthquake, which South Korea put at magnitude 3.0, was detected in Kilju county in North Hamgyong Province, where North Korea's known Punggyeri nuclear site is located, the official said.

But the US Geological Survey said it could not conclusively confirm whether the quake, which it measured at magnitude-3.5, was manmade or natural.

"The depth is poorly constrained and has been held to 5 kilometres by the seismologist," USGS said.

Previous quakes from North Korea have indicated nuclear tests by the reclusive state, the most recent earlier this month.

Tremor not as powerful as previous nuclear quakes

China's official news agency Xinhua said the epicentre was roughly at the same place as a similar shallow earthquake on September 3, which turned out to be caused by North Korea's sixth and largest nuclear test.

All of North Korea's previous six nuclear tests registered as earthquakes of magnitude 4.3 or above.

The regime's first and weakest nuclear test, conducted in 2006, generated a magnitude-4.3 quake, and the USGS measured this month's nuclear test at magnitude-6.3.

Nuclear proliferation watchdog CTBTO was analysing two seismic events in North Korea, but said they were probably not deliberate explosions.

"Two #Seismic Events! 0829UTC & much smaller @ 0443UTC unlikely Man-made! Similar to "collapse" event 8.5 mins after DPRK6! Analysis ongoing," CTBTO executive secretary Lassina Zerbo tweeted.

Tremor detected amid nuclear test tensions

Tensions have continued to rise since North Korea carried out its sixth nuclear test, prompting a new round of UN sanctions.

US President Donald Trump described Pyongyang's leader Kim Jong-un as a "rocket man on a suicide mission" at the UN General Assembly this week, and urged member states to work together to isolate the Kim regime until it halts its hostile behaviour.

The North dismissed the address as "the sound of a dog barking" and brushed aside threats by saying Mr Trump was "clearly dreaming".

The United Nations Security Council unanimously voted to impose sanctions, which cap crude oil exports to the regime at current levels, ban textile exports, and prohibit all countries from authorising new work permits for North Korean workers.

"We are done trying to prod North Korea to do the right thing, we are now acting to stop it from doing the wrong thing," US envoy to the UN Nikki Haley said after the resolution was passed.

On September 3, North Korea said it successfully tested a hydrogen bomb designed to be mounted on its newly developed intercontinental ballistic missile (ICBM).

State television described the test, ordered by Mr Kim, as a "perfect success" and a "meaningful" step in completing the country's nuclear weapons program.

It was the sixth nuclear test by Pyongyang since 2006, and South Korea estimated it was five to six times stronger than North Korea's fifth test a year ago.

Reuters