Belgian authorities on Tuesday were searching for eight migrants following their failed bid to cross to Britain by boat, police said.
Six other migrants who were in the boat, five Iranians and an Afghan, were arrested after returning to shore near the border with France, they said.
Those detained said the other eight -- including possibly two children -- came back ashore with them, but police said they had "no certainty" as to their whereabouts.
Attempts by migrants seeking to get to Britain by crossing the English Channel off France have increased in recent months, prompting increased surveillance by UK and French authorities. There have been fewer reports of bids to cross the North Sea off Belgium to Britain.
Belgian police had initially launched a sea rescue operation after a migrant suffering hypothermia was discovered near a building before dawn.
That migrant said he had been in a boat with 13 others who had left from De Panne, a Belgian coastal town just north of the French city of Dunkirk.
The five others arrested were found on a bus heading to France, leading police to switch to a search on land.
French police had been informed, Belgian officials said.