At least 27 migrants are dead after a boat capsized in the English Channel, off the coast of Calais, northern France on Wednesday, police sources told CNN affiliate BFMTV.

France's regional maritime prefecture said in a statement that a search-and-rescue operation for survivors was still ongoing in the Strait of Pas-de-Calais.

French Prime Minister Jean Castex described the incident as a "tragedy." "My thoughts are with the many missing and injured, victims of criminal smugglers who exploit their distress and misery," Castex said in a tweet on Wednesday.

French Interior Minister Gerald Darmanin is heading to Calais. He expressed "strong emotions facing the tragedy of the deaths of many migrants after a boat capsized in the English Channel."

"We can never say enough about the criminal nature of the smugglers who organize these crossings," he added in a tweet.

(With input from agencies)