TAIWAN'S President, Mr Lee Teng-hui, yesterday pledged to win international backing in three years for the return of the Kuomintang-ruled (KMT) island to the United Nations. Delivering a speech at the opening session of the National Assembly, Mr Lee also vowed to push to join more international organisations to contribute more to the peace and prosperity of the world. ''We will actively seek to join the UN,'' he said. ''We hope the international community will seriously consider this issue within three years.'' Ousted from the UN in 1971, Taiwan has been striving to regain its seat in the world body in the past few years as part of its campaign to break international isolation. The ruling KMT's main rival, the Democratic Progressive Party, which made strong gains in last December's general elections, wants Taipei to reapply for UN membership immediately. It has staged a series of demonstrations to press its demand. Mr Lee said changes in the international environment had given the island republic the opportunity to expand its external relations. He claimed Taiwan had won respect from the international community as a result of its constitutional reforms in the midst of the world tide of democracy and freedom. The replacement of military strength by economic power in world politics would help Taiwan in pushing its ''pragmatic diplomacy'', Mr Lee told the National Assembly. Taiwan has applied for membership of the General Agreement on Tariffs and Trade, and has said it wants to join several other multilateral economic organisations. Mr Lee also called on Asian neighbours to establish a system of collective security to enable disputes to be settled through dialogue. He urged the strengthening of economic and trade co-operation to reduce political disputes and called for joint efforts to develop natural resources to eliminate potential conflicts, such as the territorial claims over the disputed Spratly Islands in theSouth China Sea. In a related development, a visiting official of Taiwan's semi-official Straits Exchange Foundation (SEF), Mr Cheyne Chiu said in Beijing yesterday about 20 athletes from the mainland and Taiwan would take part in a 2,000-kilometre marathon run from Taipei to the northeast part of China via Beijing in September ''for the health of Chinese people''. The campaign, jointly organised by the SEF and its Beijing counterpart, the Association for Relations Across the Taiwan Straits, is aimed at increasing public awareness of the significance of health, said Mr Chiu during a visit to the Great Wall.