Authorities were taking extra counterterrorism precautions Friday in response to what they said was an unsubstantiated radiological threat to the city.

Officials said they had not changed the city's terror alert status in response to online chatter mentioning a truck packed with radioactive material. But police deployed extra radiological sensors on street, water and air patrols, and were stopping vehicles at checkpoints in lower Manhattan and around the city.

Deputy Police Commissioner Paul J. Browne called the measures "strictly precautionary." He said the online posts were made following a video released Sunday that featured an American member of Al Qaeda threatening foreign diplomats and embassies across the Islamic world.

"We are closely monitoring the situation," said Homeland Security Department spokesman Russ Knocke. "There continues to be no credible information telling us that there's a threat to the homeland at this time."

The FBI also said there was no credible threat.

Mayor Michael Bloomberg said the police measures were nothing out of the ordinary.

"These actions are like those that the NYPD takes every day — precautions against potential but unconfirmed threats that may never materialize," he said in a statement.