North Korea will gain "nothing" by making threats, US President Barack Obama said in the South Korean capital Seoul.

He warned North Korea of sanctions with "more bite" if it went ahead with a fourth nuclear test.

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

Stressing that the US and South Korea stood "shoulder to shoulder" in their refusal to accept a nuclear North Korea, Mr Obama said even the North’s only major ally, China, was becoming alienated by its provocative behaviour.

"Threats will get North Korea nothing, other than greater isolation," Mr 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."

Both the US and South Korea have repeatedly called on China to use its political and economic influence and exert more pressure on North Korea 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, Mr 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.

Meanwhile, a 24-year-old American tourist has been detained in North Korea for what it said was "a gross violation of its legal order".

North Korea said it had detained Miller Matthew Todd on 10 April for his "rash behaviour in the course of going through formalities for entry into the DPRK to tour it".