French police were seen driving into Italy over the weekend to return two recently-arrived migrants.
Italy's far-right Interior Minister Matteo Salvini on Tuesday hit out at President Emmanuel Macron after French police were caught committing the "unprecedented offence" of dumping migrants in Italian woods.
Salvini, also deputy prime minister and head of the anti-immigrant League party, on Monday demanded a "clear response" after French authorities admitted to returning migrants to Italy in "error".
A French police van was seen on Friday driving into Italy to return recently-arrived migrants to the town of Claviere.
"It was an error to enter Italian territory without the authorisation of the Italian police," said Cecile Bigot-Dekeyzer, the top official in the Hautes-Alpes region.
"The police had no right to enter Italian territory," the prefect said.
An outraged Salvini batted away that explanation, while France said the incident should be "kept in perspective".
"Abandoning migrants in an Italian wood can't be just a mistake or an incident," Salvini said on social media.
"What happened in Claviere is an unprecedented offence towards our country."
"Does Paris, which claims to be civil, find it normal to throw people into the woods?... We're dealing with an international shame, and Mr Macron can't pretend he doesn't know. We won't accept any excuses," Salvini wrote.
'It was a mistake'
"Let's keep this in perspective," an official in the French president's office told journalists on Tuesday.
"It was a mistake, the authorities have admitted that. There was an incursion, not planned or according to procedure, into Italian territory, where two people were dropped off."
Thousands of migrants are caught each year trying to enter France and returned to the Italian border. Last year AFP journalists saw French police dropping off migrants in front of Bardonecchia train station, in Italy.
A source close to Macron slammed Salvini's "essentially individual political exploitation" of the incident.
Italian Prime Minister "Giuseppe Conte hasn't turned this incident into proof of a crisis," the French presidency said.
"We manage our shared border together and there are occasional small regrettable incidents on both sides."
Relations between Rome and Paris have been increasingly tense in recent months, with Italy's populist government accusing France and others of failing to share the burden of the 700,000 migrants and asylum seekers that have crossed the Mediterranean to come to Italy since 2013.
French police in March sparked outrage by carrying out identity checks at Bardonecchia station, with the Italian foreign ministry summoning the French ambassador to protest.
In June, Macron criticised Salvini for closing Italian ports to the Aquarius migrant rescue boat, prompting a fresh summoning of the ambassador.