A train derailment in southern India that killed 20 people and injured up to 90 more on Saturday was an act of sabotage, a government forensic expert said on Monday.

The track from which the Hyderabad to Bangalore train derailed had been cut in two places, said KPC Gandhi, director of Andhra Pradesh's Forensic Science Laboratory.

''It was deliberate sabotage,'' Mr Gandhi said. ''We found iron particles at the spot and we're trying to find out what implement was used to cut the rail.''

He said the rail had been cut ''over a period of time,'' but did not specify how many days.

The train was travelling at about 90 kilometres per hour (55 mph) through Kurnool district when at least seven carriages overturned and a small number left the track completely.

Cigarette butts

Earlier reports said two large hacksaw-type blades had been found by police sniffer dogs on Sunday in bushes near the sheared rail.

Initially poor track repair was thought a likely cause

P Krishnaiah, spokesman of South-Central Railways, said cigarette butts were also found there and that a large rock was placed at the spot where the rail was broken in two.

Initially it was thought the generally poor state of the tracks might have been to blame for the incident.

Track failure has led to a number of serious accidents on India's railway system, one of the world's largest.

At least 118 passengers died in September near Bihar's Rafigung station when the Delhi-bound Rajdhani Express went off the tracks and several carriages fell off a dilapidated railway bridge into a swollen river.