The United Nations has temporarily closed down all of its offices in Quetta, the capital of Pakistan's south-western Balochistan province.
The decision comes after a suicide bombing just across the border in Spin Boldak in neighbouring Afghanistan.
Other UN offices along the border with Afghanistan have also been closed.
UN humanitarian co-ordinator Jan Vandemoortele said the move came after a threatening phone call was received by the UN refugee agency, UNHCR.
"Yesterday afternoon we got a telephone threat to one of our offices, the office of the UNHCR in Quetta, and it is considered as a credible threat," Mr Vandemoortele said.
"I decided... to withdraw people from the field for the time [being], and to stay home for 48 hours. The name of al-Qaeda was mentioned twice in the telephone call."
Mr Vandemoortele added that the caller had talked of an attack on UN offices.
The closures affect the local offices of the World Food Programme and the UNHCR in Balochistan.
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