Paris - French sailors operating from a frigate in the Indian Ocean have boarded what was described as a pirate mother-ship and arrested 11 pirates, the French Defence Ministry said Wednesday.

In the operation, which took place some 900 kilometres east of the Kenyan port of Mombasa, the sailors seized the 10-meter-long ship which was carrying a large reserve of petrol and two "assault skiffs," the ministry said in a statement.

The 11 pirates were arrested and transferred to the frigate, the Nivose.

They were first spotted late Tuesday when a helicopter from the Nivose helped ward off an attack on a freighter sailing under a Liberian flag. The helicopter then followed the attack boats back to the mother-ship.

The Nivose is one of eight European warships taking part in Operation Atalanta, which is aimed at halting pirate attacks on private and commercial vessels in the Indian Ocean.