By Omer Farooq BBC reporter in Hyderabad

An outlawed left-wing organisation in the southern Indian state of Andhra Pradesh has claimed responsibility for Monday's bus blast, which killed 14 people and injured several others.

The bombing of a passenger bus... is one more serious mistake in the history of the revolutionary movement

PWG statement

The People's War Group (PWG) apologised for what it calls an "unforgettable mistake".

The written apology was sent by a senior leader of the PWG, Marakapuam Bhaskar, to local newspaper offices in a remote part of the state's Warangal district on Thursday night.

It is not the first time the group has had to apologise for attacks it has carried out.

Apologies

The statement by the outlawed group said: "The bombing of a passenger bus, mistaking it to be a police vehicle, is one more serious mistake in the history of the revolutionary movement.

"We apologise to the people, to the democratic forces and to the families of the victims."

The PWG said policemen - and not ordinary citizens - had been its intended target.

Some of the injured remain in a critical condition

Its informer, it said, had failed to communicate the last-minute change in the plans of the police not to travel in that particular bus.

The group said the attack was aimed at avenging the killing of six PWG members last Sunday.

Calls to resist

The PWG also appealed to locals to resist what they called police attempts to use public buses as a form of protection from strikes by the left-wing organisation.

Monday's attack was on a state-run bus travelling near a forest in Warangal district.

The vehicle drove over one or more landmines. Gunmen then opened fire on the burning wreckage.

14 people died and 17 others were injured.

Several years after the PWG torched a train compartment, killing 46 people in 1990, the group also apologised to the victims.

And last December, the PWG killed a popular tribal leader and Congress legislator, Ragiah Naik.

The group later said it had been a case of mistaken identity and that it never wanted to kill Mr Naik.