The accident occurred early on Sunday when a Tunisian freighter rammed into a Cyprus-based vessel anchored about 30 kilometres off the northern tip of the French island.
The ship's hull was pierced and at least one fuel tank began leaking into the sea off the holiday island of Corsica.
French Environment Minister Francois de Rugy said some 600 tonnes of propulsion fuel had spilled into the sea. He condemned the "abnormal behaviour" of the Tunisian ship in a press briefing at Corsica's Bastia airport after surveying the affected area by helicopter.
Officers of France's paramilitary gendarme police force had been airlifted to the ships to investigate the incident, but it was too early to say what had happened, he added.
The spreading fuel slick. Photo: French Marine Nationale/AFP
Italian anti-pollution vessels have begun pumping the slick, Rugy said, adding that while the collision occurred near the Cap Corse marine reserve, the spill remained outside the park and moving away from Corsica. The island is known for its pristine waters and beaches.
"All means are mobilised, civil and military teams, French and Italian are working hard and doing everything so that this accident does not have ecological consequences," he said.
"The first priority is to extricate the two boats," Rugy said, adding the operation was "very complicated", with the Tunisian boat wedged into the Cyprus-based CLS Virginia.
A criminal investigation has been launched for "pollution" brought on by a "maritime accident", Marseille prosecutor Xavier Tarabeux said.
Writing on Twitter, Corsica leader Gilles Simeoni said earlier he was determined to find what caused the accident, which occurred in relatively calm seas with good visibility.
Photo: Alexandre Groyer/French Marine Nationale/AFP