Five Sri Lankan fishermen have been charged with vandalising a tsunami warning buoy just weeks before an earthquake triggered a major alert across the Indian Ocean, police said Tuesday.

Parts of the electronic floating device were removed from the buoy's deep-sea mooring off the coast last month and later recovered in the southern coastal district of Matara.

The fishermen appeared before the chief magistrate in Matara on Monday and were remanded in custody until April 30, police spokesman Ajith Rohana told AFP.

"They said they thought the device could be a nice ornament they can fit onto their boat," a court official said by telephone.

Tsunami warnings are vital to Sri Lanka, which had its south and east coasts devastated in the December 2004 tsunami when 31,000 people died in the island.

On April 11, a 8.6-magnitude earthquake off Indonesia's Sumatra island set off high-level tsunami alerts around the Indian Ocean but no large waves were created and it caused little damage.

The Deep ocean Assessment and Reporting of Tsunamis (DART) buoy, which belongs to India, is part of a wider warning system to detect unusual rises in sea levels and predict tsunamis.

"The Indian authorities complained to us that one of their buoys had been cut from the moorings," Sarath Lal Kumara, spokesman for Sri Lanka's Disaster Management Centre, told AFP.

He said police investigations led to the arrest of the five men and the recovery of some of the parts of the buoy, which cost hundreds of thousands of dollars.