A Japanese-built facility in China on Monday began destroying the largest cache of World War II chemical weapons abandoned in the country, Tokyo officials said, in a rare case of co-operation between the two countries.

The facility has been built in Harbaling in the northeast, where some 300,000 to 400,000 chemical weapons are believed to have been left behind, a government official at Japan's Cabinet Office told AFP.

Both Japanese and Chinese staff work at the facility, the official said, adding: "As our responsiblity, we will sincerely proceed with the disposal by paying great attention to the safety of local people and the environment."

Relations between Tokyo and Beijing are at their worst for years over a territorial dispute in the East China Sea, and the continuing legacy of Japan's brutal 20th-century occupation.

"Abandoning these weapons was one of the severe crimes Japanese militarist invaders committed during their invasion of China," China's official news agency Xinhua Sunday cited an unnamed Chinese official as saying.

In 1999 Tokyo and Beijing agreed to destroy the devices, with Japan providing all necessary funds, technology, experts and other resources. Originally the process was meant to be completed by 2007, a deadline later pushed back to 2012. It has since been delayed further.

The Japanese government's Abandoned Chemical Weapons Office says on its website that a total of 47,000 chemical munitions have previously been "excavated, recovered and stored".

Beijing, which regularly calls on Tokyo to show sincerity in confronting the past, urged Japan to work faster on the issue.

"The progress is still lagging far behind the plan set by China and Japan," foreign ministry spokeswoman Hua Chunying told reporters.

"China asks Japan to increase their input both in terms of personnel and materials and accelerate the destruction of chemical weapons left by Japan in China."

Japan used more than 7,300 tonnes of toxic gases to make 7.5 million weapons between 1931 and 1945, the Beijing Times said Monday, citing Japanese scholar Yoshiaki Yoshimi.