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TOKYO, Feb 4 (Reuters) - Japanese officials will travel to China later on Monday to try to find out how dumplings made at a factory there were contaminated with pesticide, sickening 10 Japanese and sparking a nationwide food scare.

A further six packets have been found with pesticide on the outside of the wrappers, as a researcher working for the Chinese government warned the problem could harm relations with Japan.

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

"Gradually, various facts are coming out but we still don't know the cause," Japanese Chief Cabinet Secretary Nobutaka Machimura told a news conference, adding that Japanese officials would go to China on Monday evening.

Chinese officials are already in Japan to investigate the case, which became public when a food trading subsidiary of Japan Tobacco Inc last week recalled products from the factory that produced the dumplings.

Other firms followed suit, while restaurants and schools pulled products from their menus, prompting health queries from about 2,000 consumers.

Chinese food quality officials said on Saturday that tests on 30 samples of frozen dumplings, ingredients and packaging taken at the factory concerned had found no pesticide, which Japanese experts have said was used widely in China but not in Japan.

A Chinese government researcher said on Sunday that Japan had been too hasty to blame China and that the criticism could harm trade relations between the two giant neighbours.

"Since the real cause is still under investigation, I don't think it wise to blame the Chinese side for it," Xinhua news agency quoted Tang Chunfeng, an expert on Japanese affairs in the Research Institute of China's Ministry of Commerce, as saying.

Sino-Japanese ties are always sensitive due to China's bitter wartime memories and present regional rivalry, but both sides seem keen to minimise fallout before Chinese President Hu Jintao comes to Japan in the spring, the first such visit in a decade.

"The Chinese response has been extraordinarily speedy," said Machimura, Japan's top government spokesman, pointing out how Beijing had immediately issued visas for the visiting Japanese officials.

Police in Japan's western prefecture of Hyogo, home to three of the 10 people who became ill, said the pesticide had also been detected on the outside of six other packages of dumplings made at the same Chinese factory.

One of the bags had two small holes but police declined to say if they suspected criminal intent.

The poison dumpling affair is also a domestic headache for Japanese Prime Minister Yasuo Fukuda, who has pledged to make his government more sensitive to consumers' needs to help bolster his sagging popularity ratings. Japanese opposition parties and media have charged that the official response was too slow.

"At the same time we feel a sense of apology to those who were harmed, we hope that this is a precious experience upon which we can draw when considering how to unify the administration of consumer (protection) affairs," Machimura said.

Chinese officials held a second round of talks with Japanese counterparts in Tokyo on Monday after agreeing on the weekend to cooperate to uncover the cause of the contamination.

China stepped up inspections after the earlier contamination cases. Last month it declared a campaign to ensure product safety a success but Vice Premier Wu Yi warned against complacency. (Reporting by Linda Sieg, Teruaki Ueno and George Nishiyama; editing by Sophie Hardach)

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