Police spokeswoman Sarah Frederickx said Wednesday the 12 men were discovered in the night from Tuesday to Wednesday and were in "good health." She said police were alerted by the truck's driver, who suspected there were stowaways in the vehicle.
Police discovered the migrants on the E34 highway connecting Antwerp in Belgium with the Netherlands. The official could not say where the migrants might have been heading. While such discoveries by police are fairly common, the danger to stowaways was highlighted last week by the discovery in Britain of the bodies of 39 people inside a refrigerated haulage container in southeastern England.
A humanitarian rescue ship operated by two French charities has arrived at a port in Sicily after nearly two weeks blocked at sea.
Ocean Viking arrived in Pozzallo on Wednesday morning, 12 days after it rescued 104 migrants from a rubber dinghy off Libya. Ocean Viking refused a Libyan instruction to land there, as it is not considered a place of safety.
A solution was reached on Tuesday when Italy said it would allow the migrants to disembark after Germany and France agreed to take them in.
SOS Mediterranee, one of the charities that operates the ship, has express relief at the agreement, but noted it was only an ad-hoc solution.
The migrants were from eight countries, including Bangladesh, Sudan and Nigeria. They included 41 minors and two pregnant women.
Spanish authorities in the Canary Islands say that four migrants have died and another is missing amid a surge of arrivals in the archipelago.
The Spanish government's delegation in Las Palmas said Wednesday that the missing and drowned African migrants were part of a group of 34 traveling in a wooden boat.
A Singapore-flagged cargo ship picked up 29 survivors and recovered the bodies of the four victims about 600 kilometers (370 miles) south of the islands.
The delegation said that two more dinghies with 158 migrants reached the Canaries on Tuesday.
About 100 kilometers (60 miles) of Atlantic Ocean separate the Spanish archipelago from the northeastern African coast.
Authorities say the islands saw in October around one third of around 1,440 migrant arrivals by sea so far this year.