SEOUL (AFP) - Negotiating partners fear that North Korea may try to keep nuclear weapons despite scrapping its plutonium-producing atomic programme, South Korea's president said.

"There is concern that North Korea might want to retain nuclear weapons that they have already produced so, in fact, they can be considered as a nuclear weapon state," Lee Myung-Bak told Japan's Kyodo News in an interview released Monday.

"This is a very serious concern we all have," he said, stressing the need for the six-party nuclear negotiations to tackle the weapons issue.

The talks group the two Koreas, the United States, China, Japan and Russia.

North Korea has recently provided a long-awaited declaration of its nuclear programme and the next round of talks will focus on ways to verify it.

The document covers plutonium production at the Yongbyon complex, which is now being disabled under a six-party deal. It does not gives any details of weapons production, an issue to be tackled later in the talks.

A senior US official said Saturday the six-party process is at "a very pivotal point" and the next round of talks would be held in Beijing as early as July 11 or 12.

US National Security Council director for Asia Dennis Wilder said the six parties need to decide on "how we will do verification of the declaration."

North Korea said Friday it could not move on to the next phase of the deal until it receives more of the energy aid it has been promised in compensation.

Japan refuses to contribute until the communist state addresses concerns about Japanese citizens it kidnapped during the Cold War years.

Lee expressed hope that Japan will start contributing, according to Kyodo, while also calling for progress on the abduction issue.

South-North relations have worsened since the conservative Lee took office after a decade of liberal rule. He has vowed to tie Seoul's economic aid more closely to Pyongyang's nuclear disarmament.

Lee reiterated he is willing in principle to meet North Korean leader Kim Jong-Il "many times" for genuine dialogue.

In a separate interview with the BBC, Lee acknowledged there had been progress in reconciliation over the past decade. But from the military perspective, not much had changed, he said.

"Less than 40 kilometres (25 miles) away, north of Seoul, North Korea still possesses thousands of long-range artillery," he said.

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