French media reported that a large number of police special forces units had rushed to a housing estate in one of Marseille's northern suburbs on Monday.

This followed reports that hooded gunmen armed with Kalashnikov rifles had opened fire on police officers in the Mediterranean port city. One account suggested that the shots had been fired into the air.

It wasn't immediately clear who the gunmen were or what their motives may have been, but the Reuters news agency quoted Marseille's deputy mayor, Caroline Pozmentier, who suggested that the incident was related to drug trafficking in the notorious La Castellane housing project.

"This battle against drug trafficking is a long-term battle," she said.

The incident came just hours before Prime Minister Manuel Valls was to visit Marseille.

It also came with security forces all over France on a heightened state of alert following last month's terrorist attacks on the satirical magazine Charlie Hebdo and a kosher supermarket in Paris, which left 17 people dead.

pfd/rg (AFP, Reuters, AP, dpa)