A small explosion damaged a US military recruiting station but caused no injuries in New York's Times Square before dawn yesterday, 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, theatres 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", but did not expose the interior of the office.

In Washington, US Homeland Security Department said there was no sign of an immediate threat to the US from the incident and a White House spokesman said there was no initial sign of any link to terrorism.

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

The one-story recruiting centre in a traffic island in the middle of Times Square invites young men and women to sign up for the US armed forces and periodically attracts anti-war protesters.

Times Square also is the site where hundreds of thousands of people gather every New Year's Eve in a celebration that has become a focus of intense police security.