Migrants trying to reach the UK in boats have set off from Belgium for the first time, a local mayor believes.
A group of 14 swam back to shore when their small vessel sank off the coast of the town of De Panne.
Mayor Bram Degrieck said eight of the group – which may include two children – are unaccounted for, but are believed to have made it back to land.
“It’s the first time to my knowledge this happens on a beach in Belgium,” he said.
Prosecutor Frank Demeester said British authorities would be consulted as part of the investigation as it was “rare” for smugglers to try to reach the UK from the Belgian coast.
Meanwhile, 11 migrants were detained by Border Force at the Port of Dover, with 10 others intercepted in a dinghy in the Channel.
Last year, nearly 1,900 people reached the UK after setting off from the French coast in small boats.
Patrols of French beaches have doubled in an attempt to curtail crossings, the Home Office has said.
“The stress is getting higher. Perhaps smugglers are trying to find different routes, other ways of getting into the UK,” Mr Degrieck said.
“It is obviously not the appropriate route to take. It is about 70km (43 miles) to the UK.
“It is obviously very distressing that smugglers are trying to use Belgian beaches,” he said.
In an interview last month Steve Reynolds, of the National Crime Agency, warned that smugglers “might try and vary the routes and have a slightly longer route in”.
“If migrants start to go further afield using longer routes the risk of fatalities obviously rises,” he said.
Mr Degrieck said Belgian police were alerted at about 04:00 GMT when one of the migrants, suffering from hypothermia, raised the alarm.
“He said he was in a group of 14 attempting to leave the coast in the direction of the UK with a small boat. That boat sank, leaving 14 people in the water.
“Six of them we found. They looked healthy. Of that party of 14, eight of them are still missing. Among those eight there is the possibility there are two children.”
“According to the people we have in custody, they say that everybody was able to reach land. Of course this information is not 100% reliable, that’s why we start a search this morning,” he said.
The search was called off with the eight still unaccounted for, but Belgian police said it believed they have left the area after making it back to land.
Of the six migrants who have been found, five Iranians were detained while taking a bus from De Panne to Dunkirk, France. An Afghan man was found in De Panne, police said.
Additionally, the French rescued six migrants after their engine failed about two miles off the coast of Calais.