BEIJING (Reuters) - China's leader-in-waiting Xi Jinping sought to reassure Southeast Asian leaders on Friday that his country wanted only peaceful relations with them, following months of growing tensions over the strategically located South China Sea.

Speaking at the opening of a trade fair in southern China for Association of Southeast Asian Nations (ASEAN) members, Vice President Xi said China's own prosperity could only be guaranteed by having good relations with its neighbors.

"The more progress China makes in development and the closer its links with the region and the world, the more important it is for the country to have a stable regional environment and a peaceful international environment," Xi said.

"Having gone through numerous vicissitudes in modern times, we are deeply aware of the importance of development and how valuable peace is," he added, according to state media.

Beijing's assertion of sovereignty over a vast stretch of the South China Sea has set it directly against Vietnam and the Philippines, while Brunei, Taiwan and Malaysia also lay claim to other parts of the region, making it Asia's biggest potential military troublespot.

At stake are potentially massive offshore oil reserves. The seas also lie on key shipping lanes.

Vietnam Prime Minister Nguyen Tan Dung is one of the ASEAN leaders attending the trade fair, held in the city of Nanning.

Xi said China - currently also involved in a dispute with Japan over a group of uninhabited islets in the East China Sea -wanted the peaceful resolution for its diplomatic arguments.

"We are firm in safeguarding China's sovereignty, security and territorial integrity and are committed to resolving differences with neighbors concerning territorial land, territorial sea and maritime rights and interests peacefully through friendly negotiations," he said.

"China's sustained development and prosperity offer an important and lasting window of opportunities to its neighbors, and promise important development opportunities to countries around the world, ASEAN countries included," Xi added.

China has resisted proposals for a multilateral code of conduct for the South China Sea, preferring to try to negotiate disputes with each of the far less powerful individual claimants.

It has also stepped up activity in the region, including establishing a military garrison on one of the disputed islands, and accused Washington of seeking to stir up trouble far from home.

Unprecedented arguments over the sea prevented an ASEAN summit in July from issuing a joint communiqué, the first time this had happened in the 10-member bloc's 45-year history.

(Reporting by Ben Blanchard; Editing by Ron Popeski)