Police said an improvised explosive device caused minor damage to the building around 3:45 a.m. (0845 GMT). The thick glass door was cracked and the lower part of its metal frame was twisted. The blast also shattered a window encasing the classic recruiting poster of Uncle Sam saying, "I want you."

The explosion occurred in the early hours of the morning, when there there are few people in the often bustling area in the heart of Manhattan.

The U.S. Homeland Security Department said there was no sign of an immediate threat to the United States from the blast, and said the FBI was taking part in a probe of it.

The targeted building sits in a traffic island between Broadway and Seventh Avenue in Times Square, sometimes called the crossroads of the world. Anti-war protesters periodically stage demonstrations there, and the building has also been hit by vandalism in the past.

Asked if there was a link to terrorism in the incident, Homeland Security Department spokeswoman Laura Keehner said the probe was still in its initial stages, but added "there is no credible information to suggest there is an imminent threat against the homeland at this time."

Police initially closed off the streets around the busy tourist and business area, but traffic was allowed through the square three hours after the explosion. Subway train operations through Times Square station were back to normal.

The targeted building sits in a traffic island between Broadway and Seventh Avenue in Times Square, sometimes called the crossroads of the world.

Capt. Charley Jaquillard, who is in charge of U.S. Army recruiting in Manhattan and works in the center, said nobody was in the building when the explosion occurred.

Asked if he thought it was a terrorist attack, Jaquillard said: "I don't know. Obviously there's some concern, but we'll see what the investigation will determine."

Speaking to Reuters behind the police cordon a block from the building, Jaquillard said the center had been the site of several demonstrations against the U.S. war in Iraq. There had also been cases of vandalism at the center in the past, he said.

Nino Reyes, 26, said he had just opened his coffee and snack stand when he heard an explosion and saw a plume of red smoke shoot up and then turn black.

He said he saw three or four people running away. (Additional reporting by Yinka Adegoke and Daniel Trotta in New York and Randall Mikkelsen in Washington; writing by Mohammed Zargham, editing by Vicki Allen)