CALCUTTA, India?Heavily armed gunmen killed four Indian police officers Tuesday at a U.S. government cultural center, and Indian officials called the slayings a “terrorist attack,” saying a Pakistan based Islamic militant group claimed responsibility.

Eighteen officers, one pedestrian and one private security guard were wounded, said state Home Secretary Amit Kiran Deb. No Americans were injured and no consulate staffers were in the building, which was closed for the day. Deb initially said five officers had died, but later said the fifth was in critical condition.

The bloody assault compounds tension between India and Pakistan, which are already on a war footing with troops massed on their common border. The United States has sought to mediate the conflict to keep efforts in the region focused on the U.S. led campaign against terrorism in Afghanistan.

Russia condemned the attack, calling it a terrorist act.