By Linda Sieg

TOKYO, Feb 5 (Reuters) - Japan's health minister on Tuesday raised the possibility that someone had deliberately contaminated with pesticide Chinese-made dumplings imported into Japan in an incident that sickened 10 Japanese and sparked a huge food scare.

Health Minister Yoichi Masuzoe's comment came as Japanese police called for close cooperation with Chinese counterparts in investigating the case, which has prompted huge Japanese media coverage and health queries from nearly 4,000 people.

The mystery of the poison dumplings is a delicate matter for sensitive Sino-Japanese ties and a domestic headache for Prime Minister Yasuo Fukuda, given criticism from media and opposition lawmakers that it took too long to alert the public.

"The circumstantial evidence suggests substantial possibility of criminality," Masuzoe told a news conference.

Investigators have yet to identify the source of the contamination in the latest furore over the safety of Chinese products ranging from tainted pet food to toothpaste.

Chinese food quality officials have said that sample tests on the frozen dumplings, ingredients and packaging at the factory concerned had found no pesticide.

Japanese experts have said the chemical is used widely in China but not in Japan, although Kyodo news agency said China banned its use this year.

However, Japanese Chief Cabinet Secretary Nobutaka Machimura told reporters it was too soon to tell if the contamination had been deliberate.

"I don't think we should hastily say that this is a (criminal) incident or the like at a stage when the facts are not yet clear," he said.

DAMPENING DIPLOMATIC FALLOUT

Japanese police declined to rule out the possibility of criminal intent. "We are investigating while keeping all possibilities in mind," a spokesman for police in Hyogo, western Japan, where three of the 10 people fell ill.

Japanese officials are in China to look into the case, which became public when a food trading subsidiary of Japan Tobacco Inc last week recalled products made at the factory that produced the frozen dumplings.

Other firms followed suit, and restaurants, schools and even the Japanese military pulled products from their menus.

Chinese officials have travelled to Japan to cooperate over the case, which both countries seem eager to keep from harming two-way ties ahead of a high-profile visit by Chinese President Hu Jintao this spring, the first such trip in a decade.

As of Tuesday, 3,732 people had made inquiries to health officials about possible illness from eating the products, but no fresh cases of illness had been confirmed, Machimura said.

Tsuyoshi Yoneda, head of Japan's National Police Agency's criminal investigation bureau, said his agency wanted Chinese police to cooperate in probing the affair.

"We are not prejudging the situation, but it is a problem that straddles both Japan and China," Kyodo quoted Yoneda as telling a joint meeting of Japanese investigators. No Japanese police are included in the team currently in China. (Additional reporting by George Nishiyama and Teruaki Ueno; Editing by Rodney Joyce)

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