Construction workers have begun a $21 million security upgrade at U.N. headquarters in New York amid word that Osama bin Laden is targeting the organization and its top leaders.

The 38-story U.N. building is one of New York City's landmarks. More than a million tourists and 100,000 official visitors come here every year.

But the World Trade Center attacks and the bombings at U.N. offices in Baghdad were a wake-up call. Security officials also remember an incident in October 2002, when a postal worker of South Korean origin climbed over the small fence in front of the U.N. headquarters and fired shots at the building to protest conditions in North Korea.

With that in mind, workers are putting up a tall spiked-metal fence that will run around the entire six square block perimeter of the U.N. complex.

But despite the tighter security, spokesman Farhan Haq says visitors will still be welcome.

?The U.N. has always appreciated the need to be accessible to all its people, to all the people of the world. That's essential to the job that we do,? he said.

Work on the security project began on the same day as an audio tape attributed to Osama bin Laden offered to pay rewards in gold to anyone killing U.N. Secretary-General Kofi Annan, or top U.S. and U.N. officials in Iraq. Spokesman Haq said the project is unrelated to the bin Laden threat, but said extra measures are being taken to protect Mr. Annan.

?We've appreciated the threats to the secretary-general, and in consultations with the host country government, we are taking steps to enhance his security,? Mr. Haq said.

Mr. Haq says the planned security upgrade will include better lighting, closed circuit television systems, vehicle barriers an electronic system for controlling access, and other unspecified items. The project is expected to take several months to complete.