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.

A man on a bicycle is believed to have thrown a 'small incendiary device' at a police officer, according to a local Eyewitness News report.

A woman in a nearby building heard a 'huge bang' and came down to investigate but was kept back by police.

'I was on the 44th floor and I could feel it,' the woman told CNN.

The scene of the blast remained closed off to pedestrians at 7am (1200 GMT) as forensic teams and bomb squad officers scoured the site.

But traffic was being allowed through Times Square, known as 'the crossroads of the world,' and subway services, initially reported to have been suspended, had returned to normal.

Dozens of emergency vehicles were at the scene, and a police helicopter hovered overhead, as curious passers-by gathered at police lines.

One window of the small recruitment station had been shattered by the blast, which also buckled the booth's door, leaving what looked like scorch marks.

Behind the shattered window, a familiar wartime 'I Want You,' recruitent poster featuring 'Uncle Sam' was exposed.

The recruitment station has frequently been the focus of demonstrations protesting the US-led military action in Iraq and Afghanistan.

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 Centre, killing nearly 3,000 people. -- AFP