Six Afghan males died when a migrant boat heading to Britain sank in the Channel early Saturday, French officials said, as a search continued to find those still missing.

 The deputy public prosecutor for the French coastal city of Boulogne, Philippe Sabatier, told AFP all six fatalities were Afghan men believed to be in their 30s.

 He added the rest of the passengers were "almost all Afghans with some Sudanese, mostly adults with some minors" and said 49 survivors were rescued -- 36 by the French coastguard and 13 by their British counterparts.

The French coastal authority Premar said up to two people were listed as missing on Saturday afternoon after the prosecutor's office had initially said between five and 10 passengers were unaccounted for.

Three French ships, a helicopter and a plane were mobilised to search the area off Sangatte in northern France, along with two British ships.

"HM Coastguard is currently assisting the French authorities, Gris Nez, in a search and rescue response to an incident involving a small boat in the Channel," a British interior ministry spokesperson said.

The spokesperson added interior minister Suella Braverman would later chair a meeting with Small Boats Operational Command, part of Britain's Border Force and created to deal with small boat crossings of the Channel by migrants.

"My thoughts and prayers are with those affected by the tragic loss of life in the Channel today," Braverman said in a post to social media site X, formerly known as Twitter.

French Prime Minister Elisabeth Borne likewise posted that her "thoughts go out to the victims" as she praised the efforts of the rescue teams.

A spokesperson for the Utopia56 humanitarian group blamed border "repression" for the tragedy, telling AFP that the difficulty of securing legal passage only "increases the dangerousness of crossings and pushes people to take more and more risks to reach England".

The boat capsized around 2:00 am local time (0000 GMT) off the northern coast of France, according to the prosecutor.  An AFP reporter in Calais saw some of those rescued disembarking from a patrol boat with emergency services on site.

 The Channel between France and Britain is one of the world's busiest shipping lanes and strong currents are common.