PARIS/BRUSSELS (Reuters) - France has protested vigorously to its northern neighbor after two Belgian policemen drove a group of illegal migrants into France and released them.

The local Belgian police commissioner for Ypres, George Aeck, appeared unrepentant, telling the broadcaster RTBF: "We didn't do it for money, this isn't human trafficking ... We only gave them a hand. We took them a little way in the direction they wanted to go."

French Interior Minister Bernard Cazeneuve telephoned his Belgian counterpart Jan Jambon on Wednesday to protest, and also had what French government spokesman Stephane le Foll on Thursday called a "frank and clear" exchange with the Belgian ambassador.

"We cannot fathom this. It must not happen again," said one French government official, who spoke on condition of anonymity. "It's contrary to the rule of law and cooperation between the two countries."

The incident, which occurred on Tuesday, began when a man driving a truck through Belgium alerted police after hearing noises in the back.

Belgian police found a dozen people inside, transferred them to a police van and drove them across the nearby frontier to the edge of a cornfield in France before their van was intercepted by French police, the French official said.

The two Belgian officers were taken for questioning and then released, a police spokesman in northeastern France said.

A second French source familiar with the case, also speaking on condition of anonymity, said the Belgian officers had remarked that it was "not necessarily the first time" such a thing had happened.

French media reported that the migrants had been trying to reach the French port of Calais to travel on to Britain, but had inadvertently hidden in a truck that was going in a different direction.

(Reporting by Emmanuel Jarry, Jean-Baptiste Vey and Pierre Savary in France, and Philip Blenkinsop and Marilyn Haigh in Belgium; writing by Brian Love; Editing by Kevin Liffey)