ROME : Prosecutors in the northern Italian city of Turin opened an investigation Sunday over an alleged intrusion by French customs officers that has caused diplomatic tensions between Rome and Paris.

A charity that helps migrants, Rainbow4Africa, has accused uniformed French customs officials of entering their office in the small Alpine village of Bardonecchia in Italy to drug-test a Nigerian passenger on a Paris-bound train.

The Italian foreign ministry summoned the French ambassador in Rome to provide an explanation on Saturday and called the alleged violation of Italian sovereignty "unacceptable."  The prosecutor's office in Turin opened a preliminary investigation on Sunday looking into allegations of abuse of power, violence and trespassing, according to the Italian agency Agi.

French Budget Minister Gerald Darmanin, who is responsible for French customs, has said that French officials were following procedures laid out in a 1990 agreement between the two countries. He declined to apologise on Sunday and implied there had been a mix-up, saying the room used by Rainbow4Africa at Bardonecchia station was a space that could be accessed by French officials under the 1990 deal.

"It transpires that between this deal and the moment that we used the room, the Italian government has granted an association that helps migrants the right to use it," he said. He added: "I will go to Italy in the next few days to explain myself in person, if necessary to the Italian government."

France has increased security on its border with Italy and carries out checks on trains and traffic coming from its southern neighbour, which is the main entry point in Europe for hundreds of thousands of African migrants each year.

French customs officials carried out a urine test on the Nigerian train passenger late on Friday which returned a negative result. Matteo Salvini, the head of Italy's far-right League, commented that Rome, "instead of expelling Russian diplomats should return French diplomats," juxtaposing the incident with the furore over the poisoning in March of a former Russian spy in Britain, widely blamed on Moscow.