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.

To calm the situation down, Darmarin nonetheless told French customs officers to suspend the practice for the time being.

However, he declined to apologize 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 neighbor, 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.

The Italian press on Sunday voiced outrage over the French incursion.

"The French are violating the borders," La Stampa said

"Migrants: Italy blocks France," was Il Messagero's take on the issue.

There is deep resentment in Italy that its fellow EU members have washed their hands of the huge migration problem it faces along its coastline.

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 furor over the poisoning in March of a former Russian spy in Britain, widely blamed on Moscow.

Migrants arrive regularly at the Bardonecchia ski station, even though trains inward to France are heavily monitored.

The first French village of Nevache is just 10 kilometers away as the crow flies.