Up to 1,000 people living in a sprawling migrant camp by Calais must leave their makeshift dwellings, the top official in France's northern Pas-de-Calais region announced on Friday.

Prefect Fabienne Buccio said the plan to move out those taking shelter in tents and lean-tos concerns about half the surface of the camp by the northern French city, known locally as the "jungle."

Officials would not say whether the move is part of a larger plan to close the squalid camp, which Buccio has suggested she wants to do.

State authorities will visit on Monday to advise those affected that they must leave.

Buccio said her agents will explain to migrants "what we expect" of them — to choose to live in heated containers set up last month on the edge of the camp that can hold 1,500 or agree to be sent to centers around France. They will be given a week to make the choice.

Refugees from Syria, Iraq, Afghanistan and other war-torn countries or dictatorships make their way to northern France in order to try to sneak across the English Channel to Britain via ferry, the Eurotunnel train or trucks. The camp on the edge of Calais now has shops, mosques, churches and schools built by migrants and volunteers.

The prefecture estimates there are about 3,700 migrants currently in the camp — lower than the more than 4,000 estimated by aid groups. There were some 6,000 people at the camp just months ago, but the prefecture has made a gradual effort to reduce the numbers. Buccio has suggested that only 2,000 migrants can remain in Calais.

"It's time to tell the migrants of Calais who live in undignified conditions and give Calais an image that isn't dignified either, that we have a solution for each of you," she said.

Tensions are mounting in Calais over the migrant situation, which some say hurts business and tourism. Migrants, in turn, are increasingly being threatened and physically attacked by armed civilians.