United Nations offices in Geneva were on heightened alert Wednesday after receiving a “credible security threat,” officials said.

The threat was received Monday evening, a statement said. No further details were available, spokeswoman Elena Ponomareva said.

U.N. Secretary-General Kofi Annan has been in Switzerland on a six-day stay, but he had already completed his visit to the Geneva headquarters when the threat was received.

The statement said the United Nations’ European headquarters – which is in a fenced and guarded park in Geneva – and U.N. agencies elsewhere in the international city had undertaken “a number of measures.”

“These include enhanced coordination with the host country authorities, greater interagency collaboration and exchange of information, and strengthened access control of deliveries, persons and vehicles,” it said.

Swiss authorities, who are responsible for the external security of the United Nations and diplomatic missions in Geneva, “are evaluating the threat on an ongoing basis.”