At least 20 people have been killed in three bomb blasts in the Indian holy city of Varanasi. Officials have not said who they believe is responsible for the attack, but Prime Minister Manmohan Singh has appealed for calm amid fears that the blasts could lead to communal violence.

Officials say two of the bombs exploded at a train station in Varanasi during the evening rush hour. The third blast took place at a Hindu temple, which was crowded with worshippers in what is one of India's holiest cities.

India's Home Secretary V.K. Duggal says it is too early to say who is responsible for the attack.

"As of now I cannot say which was the outfit responsible for this ghastly and absolutely condemnable action," said V.K. Duggal. "But certainly since one of the places attacked was a temple, so it has the potential of creating suspicion and creating tensions amongst different the communities."

Duggal said all state governments had been put on alert so that tensions between India's majority Hindus and minority Muslims do not flare into violence in the wake of the bombings. Security has also been increased at large temples.

Indian Prime Minister Manmohan Singh has also appealed for calm.

Local media report that police have defused at least six other bombs across Varanasi.

In the past, attacks at holy sites in India have led to clashes between India's Hindus and minority Muslims.

India is home to a variety of separatist groups and insurgencies, many of which would have a motive for the attack.

Three explosions rocked the capital New Delhi in October, killing more than 60. Officials blamed the attack on Islamic extremists fighting to force India to leave the disputed region of Kashmir. The predominantly Muslim region is claimed by both India and neighboring Pakistan.

Varanasi's Sankat Mochan temple, where one of the blasts occurred is located along the River Ganges, and is one of India's oldest Hindu holy sites. Officials say neither idols inside the temple nor the temple structure itself were damaged by the blast.