Russian Foreign Minister Sergei Lavrov began a mission to North Korea Thursday to try to persuade it to return to international nuclear disarmament talks but reported no immediate progress.

The North meanwhile turned up the heat on South Korea, threatening tougher steps against a joint industrial complex unless it receives more money from the venture.

Lavrov is the first high-level official to visit Pyongyang since it announced it was quitting the six-nation nuclear talks and would restart a programme to make weapons-grade plutonium.

The North was responding angrily to UN condemnation of an April 5 rocket launch which it says put a satellite in orbit. Other nations saw it as a disguised missile test.

"So far we do not expect any immediate breakthroughs," Lavrov said after meeting his counterpart Pak Ui-Chun.

"It is a difficult situation but one does not need to succumb to emotions and should concentrate on the foundation we already have," Interfax and RIA-Novosti news agencies quoted him as saying.

It is still unclear whether he will meet leader Kim Jong-Il.

North Korea's official news agency said only that the two sides "exchanged views on developing the relations between the two countries and a series of matters of mutual concern".

Permanent Security Council members Russia and China had resisted pressure for a binding UN resolution in response to the launch. But they supported a statement condemning Pyongyang and tightening existing sanctions.

Lavrov has repeatedly called for Pyongyang to return to negotiations over its nuclear programme. Russia is involved in the six-nation nuclear talks which also include the two Koreas, China, Japan and the United States.

Lavrov flies on to South Korea Friday to brief his counterpart Yu Myung-Hwan.

"North Korea must not aggravate the situation any further but abide by the UN Security Council resolution and return to the nuclear disarmament process as early as possible," Yu told reporters.

He said Seoul had been in close consultation with Moscow over the nuclear issue and the fate of a South Korean detained at the Kaesong joint industrial estate in North Korea.

Yu reaffirmed that South Korea would seek UN intervention to free the worker, detained since March 30 for allegedly criticising the communist state's political system.

The two countries held brief talks at the Seoul-funded estate Tuesday but the North again rebuffed the South's demands for access to the detainee. It presented Seoul's negotiators with a document stating its demands.

The South's Yonhap news agency said Pyongyang had warned in the document it may take "stronger measures" against South Korean firms in Kaesong unless Seoul responds favourably to its latest demands.

The North wants pay rises for its workers, new charges for land use and a review of overall contracts governing the joint venture, which was launched as a symbol of reconciliation.

In its document, according to Yonhap, it said all preferential treatment offered to South Koreans at Kaesong may be reconsidered as long as the South Korean government continues its "confrontational" policies.

The North accused Seoul of driving relations to a dangerous point by stating that it intends to join a US-led anti-proliferation initiative.

Pyongyang has previously said it would view Seoul's participation in the Proliferation Security Initiative as a "declaration of war". The initiative aims to block air and sea shipments of weapons of mass destruction.

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