The United Nations is raising its security level in Pakistan and ordering children of international staff to leave the country amid increasing violence, a UN official said Thursday.

A suicide bomber killed himself and three other people in an attack outside the Charsadda home of Awami National Party Leader Asfandyar Wali Khan, police said.

The blast is the latest in a wave of bomb attacks by Islamist militants.

Last month, a suicide bombing at the Marriott Hotel in the Pakistani capital, Islamabad, killed 55 people.

"Phase three has been approved by the secretary general," said UN information officer Ishrat Rizvi, referring to a security level that requires dependents of UN staff to leave a country.

UN Secretary General Ban Ki-moon approved the move after the agencies in Islamabad recommended it earlier this week, officials told the Associated Press.

The upgraded security level will also apply to the neighbouring city of Rawalpindi and areas around the Afghanistan border, officials said. Much of the border region, including the city of Peshawar, were already off-limits for UN families.

Officials said some families may be allowed to relocate to safer areas, such as Lahore or Karachi.

Rizvi said the United Nations would later be issuing details about the decision.

There has been growing unease about security in Pakistan despite government assurances about efforts to protect foreigners.

Pakistan's military has reported that suicide attacks have killed nearly 1,200 people — most of them civilians — since July 2007.

Work will continue

The new security level would have no impact on the UN's operations in the country, Rizvi said.

"It's a matter of only evacuating the children of international staff members, which doesn't make any difference to the work of the United Nations," she said.

About 100 of the UN's more than 2,000 staff in Pakistan are foreigners. But officials said only 20 people had children who would be required to leave the area.

Britain announced on Wednesday that it is also withdrawing the children of its diplomats from Pakistan.

The country has been under intense U.S. pressure to combat Islamic militants among the border region of Afghanistan, a known haven for al-Qaeda and Taliban militants, and Pakistani extremists who attack American and NATO troops.