UPDATE 7-Small explosion hits New York's Times Square

(Adds more from police commissioner, security video)

By Emily Chasan and Walker Simon

NEW YORK, March 6 (Reuters) - A small explosion damaged a U.S. military recruiting station but caused no injuries in New York's Times Square before dawn on Thursday, triggering a Pentagon alert for other stations across the country.

"We're treating it as if it were an incident of vandalism," Army spokesman Paul Boyce said at the Pentagon.

Times Square -- the normally bustling "Crossroads of the World" with shops, restaurants, hotels, theaters and office towers -- was largely empty when the crude bomb went off at around 3:45 a.m. (0845 GMT).

Low-grade explosives packed in an ammunition box cracked the recruiting station's thick glass door and twisted its metal framing, police said. The blast also shattered a window encasing the classic poster of Uncle Sam saying "I Want You."

In Washington, the Homeland Security Department said there was no sign of an immediate threat to the United States from the incident and White House spokeswoman Dana Perino said there was no initial sign of any link to terrorism.

The Army sent a notice to its 1,650 recruiting stations nationwide to remind recruiters to be careful, Boyce said.

New Yorkers have been on alert since al Qaeda militants used hijacked planes to destroy the World Trade Center towers in Manhattan on Sept. 11, 2001, killing more than 2,700 people. The Twin Towers were also targeted in 1993 by a truck bomb that killed six people.