Indian police said the four men, draped in shawls, sped up to the American Centre building on two motorcycles at about O615 local time (0045GMT), refusing to stop at checkpoints and shooting at police guards who returned fire.

Sixteen other guards were injured in the attack, three critically, before the attackers fled.The area has now been sealed off.

Four of the dead officers were killed on the spot. They were all from the Calcutta police or a private security agency, Group Four.

We have tightened the entire security arrangements at the US embassy and all US establishments

Police commissioner Suresh Roy

The US consul-general in Calcutta, Christopher Sandrolini, said consular staff had escaped unscathed.

Indian Home Minister Lal Krishna Advani said that a senior member of a militant group which had links with Pakistan's main intelligence agency had claimed responsibility. Mr Advani did not name the group.

Pakistan was swift to deny any involvement in the attack.

"These are totally baseless charges. As as you know, Pakistan has condemned terrorism in all its forms and manifestations," a Foreign Ministry spokesman told reporters.

Guilt claims

Two groups telephoned newspaper offices saying they carried out the attack.

One caller said he was from Harkat-ul Jehad-ul Islami (HUGI), which is active in northern India, and said the attack was in protest against "the evil empire of America".

Security has been increased at US missions in India

The other said the Asif Raza Commandos, a group named after their leader, a Calcutta criminal with links to radical Islamic groups, were responsible.

The American Centre, one of two US Government buildings in Calcutta, houses the US Information Service library, as well as the embassy's public affairs office, the press section and a wing where cultural events are held.

A spokesman for the US consulate in Calcutta, Rex Mozzer, told the BBC there was no warning for the attacks.

He refused to speculate on who the attackers were or what was being attacked - a symbol of the US Government or the Indian police.

Protests

Mr Mozzer said there had been demonstrations outside the centre, but not for several weeks and they had all been peaceful.

But India's joint commissioner of police, Suresh Roy, told the Associated Press there had been intelligence reports of a possible strike against US interests ahead of India's Republic Day celebrations on Saturday.

"We have tightened the entire security arrangements at the US embassy and all US establishments here," he said. "A general alert has been sounded. This is a serious matter. We're keeping a close watch."

Security at American missions in India had already been stepped up following the 11 September attacks in the United States.

The authorities in India also increased security in the wake of the attack on the Indian parliament last month.

West Bengal's chief minister, Buddhadev Bhattacharya, said local police had joined federal intelligence agencies in trying to track down the attackers.

But he said there was no way any US agency would be allowed to join the investigation.