Calcutta suspects rounded up

Four police officers were killed on the spot

Indian police say they have detained at least 50 people in connection with Tuesday's attack on the American cultural centre in Calcutta.

A police official told the BBC four Bangladeshi nationals were among the detainees who include an Islamic scholar.

Advani has been critical of Pakistan

India blames militants with alleged links to the main Pakistan intelligence agency for the attack by four gunmen, who shot dead five policemen and injured 20 other people outside the centre.

Pakistan has denied the accusations as "totally baseless".

The shooting comes five weeks after an attack on Delhi's parliament sparked a dangerous escalation in military tensions with Pakistan.

Islamabad has banned two of the groups which Delhi blames for the attack on its parliament and has pledged to fight extremism.

None of those detained in connection with the attack have been charged yet, the police official told the BBC.

Two Muslim clerics, believed to be have entered India illegally from Bangladesh, were arrested in the border town of Basirhat, police say.

The Bangladeshi Foreign Minister, Shamser Mobin Chowdhury, has protested to the Indian High Commissioner in Dhaka at the detention of his nationals.

Mr Chowdhury said allegations that Bangladeshi citizens were involved in the attacks were "baseless".

Claims

The US embassy and other American institutions in India are under high alert after the attack, as are the diplomatic missions of seven other countries including the UK, France and Russia.

There were intelligence reports of a possible strike against US interests ahead of India's Republic Day celebrations on Saturday, but no direct warning.

Indian police said that on Tuesday morning four heavily-armed men sped up to the American Center building on two motorcycles, refusing to stop at checkpoints and shooting at police guards who returned fire.

Four of the dead officers were killed on the spot.

Consular staff escaped unscathed.

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".

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

Indian Home Minister LK Advani did not name the group.

But he continued to criticise Pakistan, saying there had been no sign of a fundamental shift in Islamabad's support for guerrilla activity in Kashmir.

Denials

Ministry officials said Dubai-based Farjan Malik, who allegedly has links to the HUGI and the Pakistani Inter-Services Intelligence (ISI), was among those who had claimed responsibility.

Security has been increased at US missions in India

"He owned responsibility and threatened that similar attacks would take place by the group in Delhi and Gujarat," the official said.

A spokesman for the HUGI in Pakistan-controlled Kashmir denied any part in the raid, the AFP news agency reported.

The United Jehad Council, a coalition of Kashmir-based Islamic militant groups, also denied involvement, according to the Associated Press.

Calcutta Police Commissioner Sujoy Chakraborty said they had not yet been able to identify which group was involved.

No role for FBI

The shooting took place while both the US State Department coordinator for counter-terrorism and the head of the FBI, Robert Mueller, were in Delhi for talks with Indian officials.

Mr Mueller said it was too early to tell what the motive for the attack was.

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.