Authorities say an explosion at a grain silo in the eastern French city of Strasbourg has injured at least four people, and a large rescue operation is underway.

A national police spokesman told The Associated Press the injuries so far are not life-threatening, but that the overall number of victims could rise as the operation is still ongoing.

A Strasbourg fire service official said 70 firefighters are at the site of Wednesday's explosion, at a port on the Rhine River. The river marks the border between France and Germany.

Both officials said the reason for the explosion is unclear. They were not authorized to be publicly named.

In a tweet, the regional administration urged the public to avoid the area of the incident to keep out of the way of rescuers.