Two ships locked together after a collision in the Mediterranean sea

The accident occurred early Sunday when the Tunisian freighter Ulysse slammed into the Cyprus-based Virginia anchored about 30 kilometres (20 miles) off the northern tip of the French island of Corsica.

In a statement Thursday night French officials announced that the two vessels had come unstuck.

"Thanks to the combined effects of the waves and earlier attempts to free the ships, the Ulysse has broken away from the container ship Virginia," they said.

It remains unclear what caused the collision, which left a hole in the hull of the Virginia.

Some 600 tonnes of bunker fuel have leaked from the ship, officials said.

Four French and four Italian ships have been involved in operations to contain the spill and recover as much of the fuel as possible, while French authorities have launched a criminal investigation into the pollution caused.

Both cargo ships will remain on site until investigators establish whether it is safe to move them, officials said.