Two Belgian police officers were briefly detained by their French colleagues as they returned a group of migrants to French soil, an incident that raised tensions between the neighbours.
French authorities in northern France said the two policemen were stopped on Tuesday after they crossed the border near the town of Nieppe. They were transporting 13 migrants.
A spokesman for France's northern department of Nord said in a statement on Thursday that "French authorities have expressed their utter disapproval following this initiative which does not comply with the usual working practices between France and Belgium".
French interior minister Bernard Cazeneuve summoned the Belgian ambassador in France on Wednesday to express the "indignation" of French authorities, the ministry press office said.
Mr Cazeneuve also spoke by telephone with Belgian interior minister Jan Jambon. Belgium's SLFP police union, meanwhile, threatened strike action unless they can meet with Mr Jambon.
The two police officers were released after being interviewed by French police, while the migrants - including three minors - have been kept in custody for checks.
According to Belgian media, the undocumented migrants were detained on Tuesday on Belgian territory. The country's Office of Foreigners ordered them the same day to leave the country, and Belgian police took the migrants at their request to the Belgian-French border at Nieppe and dropped them off there.
In escorting one group, Belgian police inadvertently drove 50 metres into French territory.
Georges Aeck, a police captain in the Belgium town of Ypres, said the two Belgian officers drove the migrants back to the border because they "did not want to leave them by the roadside and let them walk to the border".
Belgian police union president Vincent Gilles said that the two officers were driven about 30 miles deeper into France. They were held for about four hours without a lawyer or interpreter.
He said the union has officially notified that it will strike next week unless Mr Jambon organises a meeting with police. The plan is to stop work for 24 hours, then assess their next move based on the minister's reaction.
Tensions between France and Belgium have lingered since last year's deadly attacks in Paris.
Many of the attackers had links to Belgium, and Belgian authorities have been accused of not doing enough to foil the plots.