France voiced concern on Wednesday about thousands of people who are refusing to seek shelter from hurricane Irma on the French Caribbean islands of Saint-Martin and Saint-Barthelemy.

"If there's a message that we want to get out, it's that people should protect themselves as much as possible and listen to the advice and instructions that have been given," Overseas Territories Minister Annick Girardin said in Paris.

A source close to the minister told AFP that an estimated 7,000 people had refused to seek shelter.

The two French territories have been placed on maximum alert and residents  have been ordered to stay indoors as Irma approaches as the strongest storm ever recorded in the Atlantic.

Girardin said inhabitants of Saint-Martin and Saint-Barthelemy should prepare themselves for high winds, surging sea levels and waves when the hurricane strikes on Wednesday.

"The situation is worrying because people don't know phenomenons of this scale in this part of the Caribbean," she said.

The larger French islands of Guadeloupe and Martinique are not expected to face the same intensity of winds as Saint-Martin and Saint-Barthelemy, but Girardin said residents there should also seek shelter.