"The Swiss authorities have contacted us to inform of a possible terrorist attack against United Nations premises in Geneva," the UN security service said in a note to staff.

"This attack would take place between 5th of October and the 10th of October. No information has been provided about the possible authors or the means to be used, and no specific target has been mentioned."

The security service told staff that it "is regarding this threat seriously and is taking extraordinary security measures."

However, it said that it did not plan to suspend operations at the UN building, which is known as the Palais des Nations.

UN spokeswoman Marie Heuze confirmed that the security level was "a little higher" than usual.

"The alert level varies according to the local situation and the information given by the Swiss authorities," she told AFP.

The UN offices are set in a large park which is itself ringed by a security fence.

Staff arriving at work on Thursday morning had noticed extra controls by UN security guards at the three entrance gates and a stepped up Swiss police presence on the surrounding streets.