An explosive device has caused minor damage to a military recruitment centre on New York City's Times Square.

The blast occurred before daybreak, when the centre was empty. No-one was hurt. A glass window was smashed.

Police blocked off the area. The recruiting station, located on a traffic island, has occasionally been the site of anti-war protests.

Asked if it could be a terror attack, a US Homeland Security Department told Reuters: "we're investigating."

Cars and police tape blocked surrounding streets.

The blast happened in the early hours of Thursday.

Witnesses staying at a Times Square hotel told the Associated Press news agency that they heard a "big bang" and could feel the building shake.

A plume of smoke was also visible after the explosion, they said.

At one point, subway trains passed through Times Square station without stopping, but normal service later resumed.