South Korea expressed reservations Friday about China's proposal for an informal meeting between North Korea and the United States ahead of any resumption of six-party nuclear disarmament talks.

China's visiting top nuclear envoy Wu Dawei has held a series of discussions with South Korean officials, briefing them on his trip to Pyongyang last week, a foreign ministry spokesman said.

Wu, who arrived in Seoul on Thursday, said the North supported China's three-step proposal for the resumption of the nuclear forum, which it quit in April 2009.

China wants an informal dialogue between North Korea and the United States and preliminary talks to take place before the full six-party session is resumed, he said.

"However our side expressed reservations, saying North Korea should first show a strong willingness about denuclearisation and a sincere attitue over the sinking of a South Korean warship," the South Korean spokesman said.

North Korea expressed its willingness to resume the disarmament talks to former US president Jimmy Carter, who left the country Friday after securing the release of a jailed American, state media said.

Tensions have risen sharply on the peninsula since Seoul and Washington accused Pyongyang of torpedoing the ship in March with the loss of 46 lives, an accusation vehemently denied by the North.

South Korea believes the North's overture for the talks are merely a ploy to dodge its responsibility for the sinking, Yonhap news agency said.

The six-way talks aimed at dismantling the North's nuclear weapons programme involve the two Koreas, the United States, China, Russia and Japan.

South Korean Foreign Minister Yu Myung-Hwan told Japanese reporters on Wednesday that the North should first disable its nuclear facilities and allow international monitors back to its nuclear complex.

Wu said Thursday after talks with his South Korean counterpart Wi Sung-Lac that six-party talks were still an "effective" tool to achieve peace in northeast Asia.

The Chinese envoy met South Korean Vice Foreign Minister Shin Kak-Soo and senior presidential security secretary Kim Sung-Hwan on Friday.

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US and North Korea: half a century of hostility
Former US president Jimmy Carter was heading home from North Korea Friday after securing the release of an American citizen and a pledge from Pyongyang that it wants to resume nuclear disarmament talks.

Here are key dates in the history of relations:

1945: Japan's colonial rule over Korea ends with its World War II surrender. Korea is divided at the 38th parallel between the North, backed by the Soviet Union, and the South, supported by the United States.

1950-53: After Soviet and US troops leave the Korean peninsula, the North invades the South and war breaks out. The US intervenes on behalf of the South as part of a United Nations force, and China on the side of the North.

The war leaves an estimated four million dead. The US retains a major military presence in the South.

1968: North Korean gunboats seize the spy ship USS Pueblo. The crew of 83 is detained for 11 months before being released.

1969: North Korea shoots down an American spy plane.

1976: North Korean troops murder two US army officers after a dispute over plans to trim a tree inside the Demilitarised Zone. US assembles major air and ground forces along the border.

1988: The US imposes sanctions on North Korea after putting the country on its list of nations supporting terrorism.

1989: US satellite pictures reveal a nuclear reprocessing plant at the Yongbyon complex.

1994: The US comes close to war with North Korea over its removal of spent fuel rods from the Yongbyon reactor. Ex-president Jimmy Carter defuses crisis with visit to Pyongyang, which vows to freeze and dismantle its nuclear programme in return for energy aid.

1998: North Korea fires first long-range ballistic missile.

2002: US President George W. Bush names North Korea as part of an "Axis of Evil".

The 1994 accord breaks down after US accusations that the North has a secret nuclear weapons programme using enriched uranium.

2003: North Korea withdraws from the Nuclear Non-Proliferation Treaty. Six-way nuclear talks start in Beijing in August.

2005: For the first time, North Korea states that it has nuclear weapons.

2006: North carries out test-firing of missiles to coincide with US Independence Day.

On October 9 North Korea carries out its first nuclear test, sparking international condemnation and more UN sanctions.

On December 18, six-party talks resume briefly after a 13-month hiatus.

2007: North Korea agrees in February to scrap its nuclear facilities in return for economic aid and diplomatic benefits. In July, North Korea says it has closed down its main nuclear site.

2008: In June, North Korea blows up the cooling tower at Yongbyon to show its commitment to nuclear disarmament.

In October, Washington removes North Korea from its terrorism blacklist.

2009: March-August: North Korea arrests and jails two US journalists on the border with China for illegal entry but later frees them after former US President Bill Clinton intervenes.

April 5: North launches a long-range rocket and announces it will quit the six-party talks and restart its Yongbyon complex after UN condemnation.

May 25: North conducts its second nuclear test, sparking tougher UN sanctions.

July 4: North launches seven ballistic missiles.

December 25: A US missionary is detained after entering North Korea to urge leader Kim Jong-Il to resign for human rights abuses but is released after six weeks.

2010: January 11: North Korea says it could soon return to stalled six-nation nuclear disarmament talks if sanctions are lifted.

January 25: An African American crossing the border with China detained by North Korea.

May 19: The United States "strongly condemns" an attack on a South Korean warship after investigators concluded a torpedo fired from a North Korean submarine was to blame.

June 25: The US begins a major naval exercise with South Korea in a show of force against North Korea.

August 27: Former President Jimmy Carter leaves Pyongyang with the African American pardoned by Kim Jong-Il.