JAPAN'S trade ministry said it had ordered Apple Inc's Japanese unit to explain incidents of overheating in some models of its first generation iPod nano portable music players.

The Japan arm of Apple, which recently overtook Microsoft as the world's biggest tech company by market value, must provide by Aug 4 details of the causes of overheating and show that it has taken steps to prevent future occurrences both in and outside Japan, the ministry said.

There have been 27 incidents of such overheating reported in Japan, including four cases where users suffered minor burns, said Seiji Shimagami, a spokesman for the Ministry of Economy, Trade and Industry (METI).

The ministry has repeatedly called on Apple to provide information on the preventative measures it has taken to date, he said, adding that based on Apple's response, the ministry may call for a recall of the affected models.

"Basically, warnings about the problem have not been sufficient. I think it will depend on the response we receive, but they may need to offer to exchange units that aren't faulty as well as those that are," Shimagami said.

The company said in a statement on its website that the issue had been traced to a single battery supplier, and that safety was a high priority for Apple.

"We are taking METI's letter in regard to the first generation iPod nano very seriously and are working closely with them to answer their concerns," Apple said.