Officials tightened security at the United Nations' European headquarters Thursday after Swiss authorities warned of a possible threat against the premises, a spokeswoman said.

No information was provided about who is believed behind the threat or what was specifically targeted.

The U.N.'s top official in Geneva, Sergei Ordzhonikidze, ordered the tightened security measures, spokeswoman Elena Ponomareva said. It was unclear how long the measures would remain in place.

A similar threat was made almost a year ago after Geneva police received a warning from the FBI in the United States. However, police said soon afterward that they did not believe there was a risk.

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Swiss authorities are responsible for the external security of the United Nations and diplomatic missions in Geneva.

The United Nations has been installing new protection at facilities worldwide since a truck bomb destroyed U.N. offices in Baghdad, killing 23 people — including U.N. envoy Sergio Vieira de Mello — in August 2003.

Cars entering the U.N. compound in Geneva were being searched thoroughly by security guards Thursday.

Switzerland's federal police said it informed the U.N. of the threat. However, spokesman Guido Balmer declined to reveal how police learned of the threat or if foreign authorities had provided information. He said all precautionary measures were being handled by police in the canton (state) of Geneva.

CountryWatch: Switzerland

Geneva police spokesman Eric Grandjean said the threat was against "one U.N. building," but refused to specify whether that referred to the global body's main office in the city, the Palais des Nations, or another building housing an agency such as the World Health Organization or the U.N. High Commissioner for Refugees.

Grandjean said police learned of the threat late Wednesday.