Japan's Health Minister Yoichi Masuzoe on Tuesday said he suspected that someone intentionally put pesticide in the dumplings.
He did not specify where he thought the poisoning took place.
Mr Li Chunfeng, head of a Chinese delegation that held talks in Japan on the issue, said the toxic dumplings were an isolated case and pointed to high security at Chinese factories.
'Employees work very close to one another on the production lines with security cameras over their heads,' he said.
'It would have been difficult for anyone to poison the products during the time of production and storage.'
'Considering reports made thus far, we believe the problem was not in the system but should be dealt rather as an individual case,' Mr Li told reporters before returning to China.
MR Li made an emotional plea for calm from the Japanese media, which has covered the dumplings scare extensively for a week.
'Today is the eve of the Lunar New Year in China. More than one billion people will eat dumplings tonight,' he said.
'I will eat dumplings myself tonight once I get back to Beijing.'
'I hope the Japanese media will trust the governments both of Japan and China and report on this with calm,' he said, nearly shedding tears.
'The members of my team also want to reunite with family tonight and eat dumplings.'
Thousands of Japanese have reported falling ill after eating the frozen meat dumplings, with 10 people requiring hospital treatment and major food companies recalling their products.
Japanese police said they found high levels of a pesticide, methamidophos, in dumplings made by Tianyang Food outside Beijing.
China - Japan's largest trading partner and its second biggest supplier of imported food - has said there were no signs of pesticide contamination at the plant.
Mr Li's Japanese counterpart, Shigeru Hotta, said the governments of the two countries would set up a special communication channel to stay in close touch.
'We have not reached a conclusion on a specific cause,' he said.
'We will check on the matter thoroughly.' -- AFP