Threats will get North Korea 'nothing': Obama

SEOUL - North Korea will gain "nothing" by making threats, US President Barack Obama said in Seoul Friday, warning Pyongyang of sanctions with "more bite" if it went ahead with a fourth nuclear test.

Obama's arrival in South Korea for a two-day visit was shadowed by growing signs that North Korea is making preparations for an imminent detonation at its main nuclear test site.

Stressing that Washington and Seoul stood "shoulder to shoulder" in their refusal to accept a nuclear North Korea, Obama said even Pyongyang's only major ally, China, was becoming alienated by its provocative behaviour.

"Threats will get North Korea nothing, other than greater isolation," Obama said at a joint press conference with South Korean President Park Geun-Hye.

"China is beginning to recognise that North Korea is not just a nuisance but a significant problem for their own security," he added.

Both the United States and South Korea have repeatedly called on China to use its political and economic influence and exert more pressure on Pyongyang to give up its nuclear weapons programme.

Satellite photos taken just two days ago showed additional activity at North Korea's Punggye-ri test site that the US-Korea Institute at Johns Hopkins University said was "probably related" to preparations for a detonation.

That analysis echoed recent warnings from South Korea that the North might be planning a nuclear test to coincide with Obama's two-day visit.

Asked how the international community might react, Obama said it would be necessary to look at "additional ways" to apply pressure, including "further sanctions that have even more bite".

North Korea is already subject to multiple levels of UN and other sanctions as a result of its three previous nuclear tests in 2006, 2009 and 2013.

Acknowledging there was no "magic bullet" that would bring North Korea into the international fold, Obama stressed the importance of the US and its two main Asian allies, Japan and South Korea, maintaining a united front in the face of Pyongyang's "thoroughly irresponsible" and provocative behaviour.

Washington has been deeply concerned by a diplomatic rift between Seoul and Tokyo that has partially paralysed their bilateral relationship.

Obama also made it clear that there would be no change in the US refusal to engage with North Korea until Pyongyang demonstrates a tangible commitment to eventual denuclearisation.

Washington will not just engage in a dialogue that "leads nowhere" and end up making concessions that reward North Korea for its bad behaviour, he said.

"Denuclearisation has to be on the table," he added.

President Park said a fourth nuclear test by North Korea would "fundamentally shake and alter" the entire security landscape in Northeast Asia.

"As a result, the UN would be left with no choice but to slap even stronger punishments on the North to safeguard the peace of the region and the world," she said.