(Adds Chinese vice foreign minister's comment, paras 6-7)
By Jack Kim
SEOUL, July 1 (Reuters) - The president of China, North Korea's only major ally, visits South Korea this week where the leaders of the two countries are expected to call on Pyongyang to end its pursuit of nuclear weapons, although Beijing will make sure it is not seen as taking sides.
In a visit certain to be watched carefully in Pyongyang, President Xi Jinping will be holding talks with South Korean President Park Geun-hye for the fifth time in a year, without yet meeting the North's leader, Kim Jong Un.
North Korea's nuclear and missile programme, and its plans to hold a fourth nuclear test, will dominate the agenda, officials in Seoul said.
"There will clearly be an expression of the commitment by the two leaders and their governments that North Korea's nuclear weapons will not be tolerated," South Korea's Foreign Minister Yun Byung-se told parliament on Monday.
"(The two leaders) are expected to spend considerable time discussing the North Korean nuclear and the Korean peninsula issues in depth, and we believe the atmosphere will be appropriately reflected in a joint document," Yun said.
Chinese Vice Foreign Minister Liu Zhenmin said on Tuesday the nuclear issue would be an "important topic" during Xi's talks with Park.
"Pushing for the denuclearisation of the Korean peninsula and maintaining the peace and stability on the Korean peninsula and solving the issue on the Korean peninsula through peaceful means has been China's goal for many years," he told reporters.
China is usually very guarded in its opinion on North Korea's nuclear programme but Pyongyang's three nuclear tests and several rounds of sabre rattling have tested Beijing's support.
In May, Seoul said South Korea and China had agreed at a meeting of their top diplomats that recent nuclear activity by North Korea posed a serious threat to the peace and stability of the region and Pyongyang must not conduct another nuclear test.
Xi, however, is unlikely to step much beyond Beijing's stated position calling for a negotiated solution to the issue through talks that involve the United States, while urging all players to refrain from actions that will further escalate tensions.
Beijing has backed U.N. sanctions imposed on the North, but is also not expected to upset its balanced approach towards the two Koreas.
Xi is also courting stronger economic and diplomatic ties with South Korea, a major trade partner, and his two-day visit includes meetings with business leaders of Asia's fourth largest economy, including executives from Samsung, LG and Hyundai Motor.
North Korea has sent a flurry of mixed signals over the past two days which has shifted some of the spotlight from Xi's visit. It tested two short-range missiles on Sunday in violation of a United Nations ban. On Monday, it said it would put two American tourists on trial for crimes against the state.
Pyongyang also came up on Monday with a fresh proposal for peace with the South, with which it is still technically at war. It offered to stop military drills as early as this week, which would coincide with Xi's visit, in return for the suspension of annual South Korea-U.S. military exercises.
CHINA NOT THE MAIN PLAYER
Despite U.S. and South Korean pressure, China is likely to maintain that it is in no position to ensure that the North give up its nuclear arms.
"The main player in this is not China, but the other two countries - North Korea and the United States," said Li Changhe, a former senior diplomat who now works for the government-backed China Arms Control and Disarmament Association.
"China is in there to push talks, getting those two to sit down together. But the problem is neither side really listens to us. We're stuck in the middle."
North Korea's Kim, who took power following the sudden death of his father in December 2011, has maintained contact with Beijing through high-level visits by officials from Pyongyang. However, the closest Kim came to communicating with Xi was through a letter handed to the Chinese leader last year through an envoy. His father Kim Jong Il went six years asserting his leadership domestically before travelling to China for the first time.
Xi, who is due to arrive in Seoul on Thursday in his first visit to South Korea since taking office last year, is reciprocating Park's visit to China a year ago.
The Asian nations have one of the world's largest commercial partnerships, with two-way annual trade at nearly $230 billion. China is South Korea's biggest trading partner.
South Korea is also one of the few major economies that runs a surplus with China, to the tune of $63 billion last year, thanks to exports of cars, smartphones, flatscreen TVs, semiconductors and petrochemicals.
China's trade with North Korea was just over $6 billion in 2012, according to South Korean government data.
North Korea knows that its nuclear capability is the key reason it commands world attention, former Deputy U.S. Secretary of State Richard Armitage said.
"My belief is that North Korea wants to be left alone, with some modest economic opening that they control," Armitage said at a recent forum in Beijing. "They don't like China much more than they like the United States." (Additional reporting by Ju-min Park and James Pearson in SEOUL and Sui-Lee Wee and Ben Blanchard in BEIJING; Editing by Tony Munroe and Raju Gopalakrishnan)