Media playback is unsupported on your device Media caption The explosion did not cause a fire at the chemical plant, officials said

At least five people have been killed and 12 injured in an explosion at a chemical factory in central Japan.

The blast occurred in the afternoon at the plant run by Mitsubishi Materials in Yokkaichi city, Mie prefecture.

Maintenance crews were cleaning out a heat exchanger used in the production of silicon products when the blast happened, officials said.

Some reports suggest the cleaning water mixed with chemical residue inside the exchanger, producing a hydrogen blast.

The plant manufactures polymers and uses a number of highly volatile chemicals including hydrogen and chlorine.

Industrial accidents

Thursday's blast at the plant - some 350km (220 miles) west of Tokyo - killed five of the maintenance workers on the spot.

The number of injured has ranged in reports from 12 to 17.

Emergency officials said there had been no fire and the situation was quickly contained.

A similar non-fatal accident happened at the same plant two years ago, the BBC's Rupert Wingfield-Hayes in Tokyo reports.

Deaths from industrial accidents in Japan have steadily declined over the last 20 years - from around 2,500 a year in 1990 to just over 1,000 a year in 2010.

This is still considerably higher than in many Western countries, but is perhaps explained by Japan's very large manufacturing sector, our correspondent says.

Mitsubishi Materials makes a range of automotive, electronics, and construction and engineering products.