Four gunmen on motorcycles opened fire Tuesday morning outside the American Center in the city, formerly known as Calcutta, killing at least four Indian police officers and wounding 21 other people. 

Indian Home Minister L. K. Advani said later that a group -- which he did not name -- with links to Pakistan's powerful Inter-Services Intelligence (ISI) agency had claimed responsibility for the morning raid. 

"As you know Pakistan has condemned terrorism in all its forms and manifestations," he said. 

The attack comes less than six weeks after a deadly assault on the Indian Parliament in New Delhi. 

That attack, which India blames on two Pakistan-based Islamic militant groups it says are backed by the ISI, has put the two South Asian nuclear rivals on a war footing -- together massing close to a million troops along their border. 

India and Pakistan have fought three wars since independence in 1947 -- two of them over the disputed Muslim-majority region of Kashmir. 

Manhunt
Following the Kolkata attack police have launched a massive manhunt for the gunmen who opened fire from under their shawls with automatic rifles as they rode their motorcycles to the center at about 6:30 a.m. (0100 GMT) Tuesday. 

They shot at police officers on patrol outside the center and then sped away before security forces had a chance to react, Indian officials said. 

No U.S. center staff were inside the building at the time of the raid and no Americans were injured, officials from the U.S. Embassy in New Delhi told CNN. 

Of the 21 people wounded, 10 were armed guards, 10 were bystanders, and one was a security staffer. 

Pools of blood, upturned chairs and pieces of clothing lay scattered on the footpath outside the building and bullet holes scarred the security wall. 

Police cordoned off the area after the attack -- believed to be the first against an official U.S. facility in India -- creating chaos in the heart of India's most densely populated city. 

Security has been strengthened at U.S. installations and diplomatic enclaves across the country including the U.S. Embassy in New Delhi. 

Indian police say they believe the attack was planned to coincide with a change of shift of the center's guards when the men are bunched together awaiting their replacements. 

The raid comes as senior U.S. officials are in India for talks on counter terrorism

The officials include FBI Director Robert Mueller and Francis X. Taylor, the Bush administration's coordinator for counter-terrorism

It was the first meeting of the group since the September 11th terror attacks in the United States. 

American officials have offered U.S. assistance to Indian investigators. 

While there have been demonstrations outside the center in the past, U.S. consulate spokesman Rex Moser told CNN he was not aware of any threats to the center. 

But Joint Commissioner of Police Suresh Roy said intelligence reports have hinted at a possible strike against U.S. establishments in India in the run-up to the country's Republic Day celebrations this Saturday. 

Last June, four men were arrested on suspicion of planning to bomb the U.S. embassies in New Delhi and the Bangladeshi capital, Dhaka.