US President Barack Obama warned Thursday that North Korea must show "seriousness of purpose" before six-party nuclear talks can resume, saying he was not interested in simply "going through the motions."

Obama, speaking ahead of the G20 summit here, also warned the Stalinist state that it would deepen its isolation by refusing to halt its nuclear programme and said Washington would never waver in its defence of South Korea.

"There will be an appropriate time and place to re-enter into six-party talks but we have to see a seriousness of purpose by the North Koreans," Obama said at a press conference with South Korean President Lee Myung-Bak.

"We are not interested in just going through the motions with the same result," Obama said, adding he and Lee were in "complete agreement" on policy towards Pyongyang.

Six-party talks on North Korea's nuclear programme are currently stalled, with the latest impediment to a resumption being the sinking of a South Korean warship that was blamed on the isolated Stalinist state.

Pyongyang left the talks in April 2009 and conducted its second nuclear test a month later.

The North has indicated a willingness in principle to return to the talks, which are chaired by its ally China and also involve South Korea, Japan, China, the United States and Russia.

Obama also spoke about the standoff at a US base about 50 kilometres (30 miles) from the heavily fortified demilitarized zone (DMZ) between the two Koreas.

"Pyongyang should not be mistaken: the United States will never waver in our commitment to the security of the Republic of (South) Korea," Obama said, as he met US troops on Veterans Day.

"The alliance between our two nations has never been stronger, and along with the rest of the world, we have made it clear that North Korea's continued pursuit of nuclear weapons will only lead to more isolation and less security."

But Obama also told North Korea that if it lived up to international obligations on its nuclear programme, it could expect rewards from the international community to help lift its people out of "crushing poverty."

The US leader, on the third leg of a four-nation Asian tour, also declared that the 1950-53 Korean War, which history records ended in a bloody stalemate, was a victory for the anti-communist forces, as it had paved the way for South Korea's stunning economic emergence.

Obama, talking to currently serving US troops and ageing veterans of the Korean conflict, paid tribute to the 37,000 American troops who died in what is often termed the "Forgotten War".

"Because the Korean War ended where it began geographically, some used the phrase 'Die for a Tie' (draw) to describe the sacrifice of those who fought here," Obama said at the US Yongsan base.

"But as we look around at this thriving democracy and its grateful, hopeful citizens, one thing is clear.

"This was no tie. This was a victory. It was a victory then, and it is a victory today."