President Francois Hollande said Wednesday that the situation on the French Indian Ocean island of Mayotte, where hundreds of foreigners have been kicked out of their homes, was "extremely worrying".
Rising anti-migrant anger has seen groups of up to 100 men gather in villages with lists of houses belonging to foreigners and going from door to door forcing them to leave.
This weekend another 500 foreigners, mostly from the neighbouring archipelago nation of the Comoros, were expelled from their homes.
"We are in a situation which is extremely worrying," Hollande said in a statement through his spokesman Stephane Le Foll on Wednesday.
In the island capital of Mamoutzou, a young woman, Mounira, told AFP she was kicked out of her home on Sunday with her husband and three daughters.
"I have a 10-year resident's visa, a permanent contract job, my rent is paid -- I don't understand," she said.
She has been fortunate to find shelter with an ex-employer, but many hundreds have been left in the open in the capital's main square, looked after by the Red Cross and volunteers.
For months, anti-foreigner groups have been whipping up hostility to migrants with leaflets accusing them of "clandestine immigration" and "daily thefts, assaults and murders".
Police have tried to prevent violence, but have been unable to stop the pressure mounting on foreigners -- including many who are living on Mayotte legally -- who have fled their homes in fear.
A local rights group, Cimade, said Wednesday that more than 1,000 foreigners have been forced from their homes this year.
Mayotte opted to remain under French rule when the other islands in the Comoros archipelago chose independence in 1975.
Although there have been protests demanding more investment from France, it remains much wealthier than the neighbouring islands and more than 40 percent of its population is thought to have been born abroad.
The government returned 18,000 people to the border last year, with mounting anger over the pressure on schools and the health system.
"With the immigration that is coming from the Comoros, the status quo is unacceptable... it's an extremely dangerous subject on which we must act," said Le Foll in Paris.
The French government earlier said it had mobilised more than 800 police to prevent further attacks and expulsions, and another 76 to help tackle illegal immigration.