A Chinese research ship has been forced to leave hotly disputed oilfields in the South China Sea after diplomatic protests by Hanoi. Vietnamese government sources said yesterday that patrols spotted the Exploration No 8 entering the disputed Wa'an Bei area with two supply vessels about two weeks ago, sparking immediate reaction by Hanoi. All three vessels were escorted without incident from the area off the southeast coast, which is marked by several large submerged reefs forming the tailend of the Spratly Islands. They were last reported to be heading towards Malaysian waters to the southeast and were being tracked by military planes understood to be from Kuala Lumpur, sources said. Hanoi has yet to comment officially but claims exclusive rights to explore and develop the fields it calls Tu Chinh and which it insists lie within its 200 nautical-mile economic zone. The appearance of the research vessel alarmed authorities in Hanoi, who said it was the first time China had returned to the area since American commercial interests intensified last year. Beijing awarded development rights to obscure Denver firm Crestone which sold the claim to US oil giant Benton for more than US$15 million (HK$116 million) last September. This was shortly after Vietnam signed a similar deal with the rival Conoco operation. Neither company has taken active steps to explore the area close to some of Vietnam's most promising oilfields being developed by British, Malaysian, US and Japanese firms. Benton is the only firm to have a contract with China in the area but spokesmen have previously vowed not to do anything to inflame an already tense situation. The appearance of the ships came about a month after Beijing forced Vietnamese fishing boats from the Paracel Islands off Hainan Island. Vietnamese troops and naval personnel occupy more Spratly reefs than any other state but China holds all of the Paracel group, whose waters are still patrolled by Vietnam. Tensions in the South China Sea have continued to simmer this year with Hanoi fiercely objecting to Chinese gas exploration off its central coast. Vietnam and the Philippines have also expressed alarm at the presence of Chinese frigates further east.