Security was stepped up at the United Nations headquarters as a precaution on Tuesday following a bomb threat, a spokesman said.

"The New York Police Department received an unconfirmed bomb threat so the U.N. stepped up security as a precautionary measure," said U.N. associate spokesman Farhan Haq.

According to a U.N. diplomat, the U.S. Customs and Immigration Service received an anonymous phone call saying a taxi would arrive at U.N. headquarters at 10 a.m. with two backpacks filled with grenades. The threat was relayed to New York police, said the diplomat, who spoke to The Associated Press on condition of anonymity because details of the threat were not released.

Just before 10 a.m., a line of about 10 New York City police cars, their lights flashing, were on First Avenue just outside U.N. headquarters, which overlooks the East River. At U.N. entrances, security staff were checking the bags of all U.N. staff. Sniffer dogs, permanently based there, were also on patrol.

By early afternoon, no incident had been reported.

The U.N.'s landmark 39-story headquarters building was reported to be a terrorist target following the 1993 bombing of the World Trade Center. The United Nations and former Secretary-General Kofi Annan have been condemned by Usama bin Laden, the suspected mastermind of the Sept. 11, 2001, suicide hijackings.

The U.N.'s headquarters was evacuated after the 2001 terrorist attacks, and again the following day, and mail delivery was halted for several weeks in the fall of 2001 because of an anthrax threat.