Japanese authorities are investigating reports of a fire involving one of Apple's iPod digital music players.

The 2GB iPod Nano emitted sparks and ignited when the built-in lithium ion battery apparently overheated while being recharged.

The owner called the local fire department, local press reports said, but no injuries were reported.

Apple apparently learned of the incident shortly afterwards, but did not inform Japan's Ministry of the Economy, Trade and Industry until March, according to reports.

The ministry issued a brief report on the fire, which took place on 8 January near Tokyo, Nikkei Business Press reported.

Shops in Japan sold more than 400,000 units of the model affected by the fire, which was identified by the product code MA099J/A, according to the ministry.

These products were sold in Japan between September 2005 and September 2006. It is unclear whether they were also sold in other countries.

The iPod Nano's battery is sealed inside the case, and is not designed to be removed or replaced by the user.

Reports of notebook PC battery fires and explosions forced Sony to spend more than $400m on a recall in 2006.

Last month, South Korea's LG Electronics announced that it had stopped making a popular laptop after two reports of fires or explosions related to its battery. The exact cause of the battery explosion remains unknown.