Gunmen on motorcycles sprayed automatic gunfire outside the U.S. government cultural center on Tuesday, killing four officers and wounding at least 20 people, police and U.S. Embassy officials said.

Draped in shawls, four attackers on two motorcycles drove up to the American Center in the heart of Calcutta at 6:30 a.m., shot at officers and fled, said the U.S. Embassy in New Delhi. There were no reports of Americans injured, and no U.S. staff members were in the four-story building, said embassy spokesman Gordon Duguid.

He said 20 people were injured, but state home secretary Amit Kiran Deb said the wounded included 11 state police officers, nine pedestrians and one private security guard.

A senior home ministry official told The Associated Press that a man phoned police in New Delhi and said the attack had been carried out by Harkat-ul Jehad-e-Islami. The Pakistan-based militant group fought the Indian army in Kashmir in the 1990s, but is no longer active. The claim could not be immediately verified.

The attack comes slightly more than a month after an assault on the Indian Parliament left 14 people dead. India blamed two other Pakistan-based Islamic militant groups for the attack, which touched off a diplomatic clash that put India and Pakistan on a war footing.

A pool of blood could be seen outside the American Center. Spent AK-47 bullets littered the street.

"They used high-caliber automatic weapons. It is a terrorist attack but we cannot yet identify which group was involved," said Calcutta Police Commissioner Sujoy Chakraborty.

He said the attack occurred when the police officers on the night shift were changing guard. The wounded were taken to a government hospital. The attackers were not hurt, he said.

In the Indian capital, a high alert was issued at the U.S. Embassy and other American institutions in New Delhi.

Joint Commissioner of Police Suresh Roy said there were intelligence reports of a possible strike against U.S. establishments in India as part of an "unprecedented threat" during India's Republic Day celebrations this coming Saturday.

"We have tightened the entire security arrangement at the U.S. Embassy and all U.S. establishments. A general alert has been sounded," Roy said in New Delhi. "This is a serious matter. We are keeping a close watch."

Security has been tightened in recent months at the American Embassy in New Delhi, consulate buildings and other offices across the country to prevent possible attacks by Islamic guerrillas fighting to separate the Himalayan region of Kashmir from Indian control.

The American Center, one of the two U.S. government buildings in Calcutta, houses a popular library, the embassy's public affairs office, a press section and a wing for cultural programs.

The center is one of the landmarks of Calcutta, the eastern metropolis that is the capital of the communist-run West Bengal state. Anti-American protests by communists and labor unions are often held there, but this is the first violent attack against an American compound in India.

The attack came a day after Indian and American officials held the first meetings of the Joint Working Group on Counterterrorism in the aftermath of the Sept. 11 terror attacks in the United States.

U.S. Ambassador at Large Francis X. Taylor, President Bush's coordinator for counterterrorism, and FBI Director Robert Mueller were in India to meet with officials.