Apple is investigating an incident in which one of its popular iPod devices apparently shot out sparks while being recharged by its owner in Japan.

A Japanese Government official was quoted as saying that the incident, which involved one of the older iPod Nanos, took place in January in Kanagawa Prefecture, southwest of Tokyo.

The owner was not hurt, the official said.

The Japanese trade and industry ministry has instructed Apple to find out the cause of the fire, which is thought to have been a result of a defective lithium ion battery - a component that has been known to overheat in other electronic devices.

About 425,000 iPods identical to the affected device, which has the model number MA099J/A, were shipped between September 2005 and September 2006, the Japanese official said, though it was not clear how many had been sold. The model has since been discontinued, he added.

A spokesman for Apple, which reported the incident earlier this month, was not immediately available for comment.

If confirmed, it would be the second time an iPod Nano has caught fire unexpectedly. In October, an airport worker from Georgia, in the US, claimed that his one-and-a-half-year-old iPod Nano caught fire while in his pocket and burned for 15 seconds. Apple is still investigating the incident.

Lithium ion batteries, which are favoured by the manufacturers of electronic devices because they are lightweight and retain their charge for longer than other batteries, have been involved in a number of product recalls because of problems associated with overheating.

In the past two years, more than 10 million lithium ion batteries for laptop computers have been recalled following concerns they were defective, including those in machines made by Dell, Apple, Toshiba, Lenovo, and Sony.

In February, 30,000 Californian authorities recalled 30,000 controlled remote-controlled helicopters after the lithium batteries inside were found to catch fire during recharging and in one case, mid-flight.

In August, 46 million batteries made for Nokia phones by Matsuhita, the owner of the Panasonic brand, were recalled after they were found to overheat once they had been recharged a couple of hundred times.

The problem is thought to result from small particles of dust that are leftover from the manufacturing process and cause the batteries to overheat in certain circumstances.

Last year the US Consumer Product Safety Commission expressed "concern" about lithium ion batteries, and said it would consider taking action against companies that recalled products if they didn't ensure their batteries were safe.

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