Indian police hunt gunmen

Investigators at the scene. U.S. Embassy officials said the motive for the attack is unclear

KOLKATA, India (CNN) -- Indian police have launched a manhunt for two unidentified gunmen who opened fire on the American Center in Kolkata (Calcutta), killing at least four Indian police officers and injuring 21 other people.

The center has been cordoned off and security has been strengthened at U.S. installations and diplomatic enclaves across the country including the U.S. Embassy in New Delhi.

Officials say the assailants, wearing shawls, rode up to the center on a motorcycle at about 6:30a.m. (0100 GMT) Tuesday.

From under the shawls, they produced AK-47 rifles and shot the police officers on patrol outside the center.

The gunmen then sped away before security forces had a chance to react, Indian officials said.

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

Of the wounded, 10 were armed guards, 10 were bystanders, and one was a security staffer. The injured have been taken to hospital for treatment, officials said.

No one has claimed responsibility for the attack.

No motive

AUDIO CNN's Satinder Bindra reports on the attack. 1.48 MB / 2:20 min WAV sound   MORE STORIES Police on alert in New Delhi     CNN.com Asia More news from our Asia edition

The American Center is in a crowded downtown area of Kolkata and houses offices of the U.S. consulate, a cultural affairs center and a library.

"The U.S. consulate is in a separate location and was not affected by events this morning," spokesman Rex Moser told CNN.

Moser added that he was not aware of any threats to the center.

U.S. Embassy officials said it was unclear what the motive for the attack was.

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.

Security issues

The incident coincides with a visit to India by FBI Director Robert Mueller and U.S. Ambassador at Large Francis X. Taylor, President George W. Bush's coordinator for counterterrorism, to discuss security issues.

On Monday, Indian and American officials also held the first meetings of the Joint Working Group on Counterterrorism, the first since the September 11 terror attacks in the United States.

Security has been tightened in recent months at the U.S. Embassy in New Delhi, consulate buildings across the country and other offices to fend off possible attacks by Islamic guerrillas fighting opposed to Indian rule in disputed Himalayan region of Kashmir.

In a Reuters news agency report, Delhi Police Joint Commissioner Suresh Roy said authorities had received intelligence of a planned terrorist attack ahead of the country's Republic Day Celebrations scheduled for this weekend.

"Threats have been coming from various militant groups, quite frankly it is unprecedented," he was quoted by Reuters.

Tensions high

Security has been beefed up at the U.S. Embassy

Tensions have been high between India and Pakistan in recent weeks.

India -- the target of two terror high-profile attacks against government buildings in October and December -- blames Pakistan for backing cross-border terrorism.

Both countries have massed hundreds of thousands of troops along their common border in eastern Pakistan and northwestern India.

Kolkata, in eastern India, is in the communist-run state of West Bengal state, and has been scene to frequent protests against U.S. interests.