PARIS (AP) — A boat carrying an unknown number of migrants across the English Channel sank off the French coast Tuesday, prompting an extensive rescue operation that has saved 18 people so far, French authorities said.

It is unclear if any migrants died in the sinking.

French authorities mobilized five vessels and a Belgian helicopter nearby to help with the rescue after the migrant boat was reported in distress off the coast of Dunkirk, according to the regional French maritime agency.

It said in a statement that 18 people were rescued and are receiving treatment in hospitals in Calais and Dunkirk.

An official with the agency said the rescue operation is still under way, and an investigation into the cause of the sinking was opened by the Dunkirk prosecutor.

French maritime officials routinely rescue migrants trying to cross the Channel and warn against the risky crossing.

Despite joint police efforts on both sides of the Channel, migrants have long used northern France as a launching point to sneak into Britain, and the issue has long strained relations between the neighbors. Last year French authorities said at least four people died trying to use small vessels to cross the Channel, known for high winds, strong currents and heavy maritime traffic.