KABUL (Reuters) – The United Nations will evacuate 900 of its international staff from Afghanistan starting Thursday, citing security concerns, a UN source in Kabul said.

Taliban militants killed five foreign UN staff in an attack at their international guest house in the capital on October 28.

"The United Nations is reducing the number of its international staff from 1,300 to 400, effective immediately," the source told Reuters.

The UN source said the world body would determine how much staff was necessary to run on-going programs.

"We will be looking at bringing people back as soon as the security situation allows and secure accommodation is found," the source said. "Instead of having people dotted around the city, they will be consolidated."

A UN statement emailed to Reuters said it would not make details of the new measures public, adding: "It is expected that they will involve short-term relocations for some staff while additional security is being put in place."