The United Nations is evaluating the security situation in Afghanistan on "an hour-by-hour basis" and moving some staff to the capital Kabul, but is not evacuating anyone from the country, the U.N. spokesman said on Friday.

The world body had "a very light footprint" in some areas taken by the Taliban, spokesman Stephane Dujarric told reporters. It has about 3,000 national staff and about 300 international on the ground in Afghanistan.

"The current situation is that we're evaluating, literally on an hour-by-hour basis, the security situation both in Kabul and in other locations. There is no evacuation of U.N. staff going on," Dujarric said.

Taliban insurgents Afghanistan's second- and third-biggest cities as resistance from government forces crumbled, stoking fears that an assault on the capital Kabul could be just days away.

The United States is sending in about 3,000 extra troops to help evacuate U.S. Embassy staff, Britain is deploying about 600 troops to help its citizens leave while other embassies, including those of the Netherlands, Germany and Norway, and aid groups said they were also getting their people out.

"We obviously assessing different places where to lighten the footprint," Dujarric said.

"There is always contingency plans for the best case and for the worst case. At this point we are remaining in Kabul...we have footprints in other parts of the country to fulfill our mandate and help civilians and alleviate the suffering for civilians as much as we can," he said.

Since the start of the year nearly 400,000 people have been newly displaced by conflict across Afghanistan, the United Nations has said. Dujarric said 10,350 internally displaced people have arrived in Kabul between July 1 and Aug. 12.