A grain silo exploded yesterday morning in the eastern French city of Strasbourg, injuring four people, according to local authorities.

More than 100 firefighters and 60 fire engines were deployed to deal with the incident in the city's river port district, the Lower Rhine prefecture said.

Three of the injured were airlifted to hospital in a very serious condition.

A fire resulting from the blast was brought under control by early afternoon, the prefecture said.

Maintenance work had been taking place on the silo at the time of the blast, prefecture secretary general Yves Seguy told press.

"Obviously cereal dust is highly explosive, but the usual safety measures had been taken," Seguy said. "So, for the moment, I cannot give you any more details on the real cause of the disaster."

Authorities said a 200-metre perimeter around the site had been secured and asked residents to avoid the area.

An eyewitness who had been working atop a crane near the site told local newspaper Dernieres Nouvelles d'Alsace that he had heard an explosion and seen an enormous fireball and projectiles flying through the air.

The newspaper quoted the head of agriculture cooperative Comptoir Agricole as saying that the explosion took place in a 40,000-tonne capacity grain silo belonging to the coop.

It was used for storing grain that was going to be loaded onto barges for transport on the river Rhine, Comptoir Agricole president Marc Moser told the newspaper.

One of the injured was a member of the cooperative's staff and the other three were working for a contractor that was carrying out maintenance work on the facility, Moser was quoted as saying

.