Police cordoned off a section of New York's Times Square early on Thursday after a small explosion struck near a military recruiting station, officials said.
A New York Police Department spokesman said only that there had been a minor explosion, that there were no injuries and that police were investigating the cause of the blast. He said he did not know if the explosion was man-made.
Local reports said the explosion struck around 4 am (0900 GMT) and that subway services to the square, one of the city's main transport hubs, had been suspended, although officials were not immediately available to confirm this.
New York has been on a heightened state of alert ever since the attacks of September 11, 2001, when hijackers plowed two passenger airlines into the World Trade Center, killing nearly 3,000 people.