The United Nations said Monday it was withdrawing its international staff from northwestern Pakistan because of the security situation there.

"They will be relocated. Immediately," Ishrat Rizvi, a UN spokeswoman, told AFP.

UN Secretary General Ban Ki-moon had declared a security condition known as Phase IV in the Northwest Frontier Province and Federally Administered Tribal Areas (FATA), a UN statement said.

"The decision has been taken bearing in mind the intense security situation in the region," the statement said.

Phase IV is the UN's second-highest security category. Under the restriction, local UN staff can continue to work but expatriates are limited to emergency operations, the UN said.

Late last month the UN's World Food Programme (WFP) temporarily closed distribution centres serving more than two million people in the northwest because of security fears. The closure affecting the Swat Valley region followed a suicide bombing of the WFP compound in the capital Islamabad. The attack left five aid workers including an Iraqi national dead.

Rizvi could not immediately say how many foreign staff would be evacuated. "We don't have big numbers of international staff in that region," she said.

Billi Bierling, spokeswoman for the UN's Office for the Coordination of Humanitarian Affairs in Pakistan, said the new restrictions will largely affect long-term development projects, not emergency operations currently underway in both the Swat Valley and FATA.

Almost all the WFP food distribution centres that were temporarily closed in Swat have reopened, she said.

They have been giving out supplies for those displaced this year by fighting in that area between the army and Taliban, which forced around two million people to flee.

Further south, UN agencies are working through local partners to distribute food, blankets and other essentials to people displaced by a major military offensive that began last month in South Waziristan, part of FATA.

Up to 250,000 people have fled the offensive in that tribal region on the Afghan border and moved to neighbouring Dera Ismail Khan and Tank, a government official said Sunday.

But even before the Phase IV decision, security concerns had prevented the deployment of international staff to Dera Ismail Khan, Tank, or South Waziristan, Bierling told AFP.

Taliban and Al-Qaeda-linked extremists have carried out a two-year campaign of attacks in Pakistan that have killed more than 2,000 people.

On Monday a bomber on a motorbike killed 20 people in the garrison city of Rawalpindi, neighbouring Islamabad.