The Taliban has taken over the United Nations office in the southern city of Kandahar and locked up all UN communications equipment in Kabul.
UN spokeswoman Ms Stephanie Bunker said the Taliban had also taken over some Kandahar offices of non-governmental organisations, which provide emergency relief services in the country.
Most UN activities have been disrupted or have ceased, she said.
The UN has asked the authorities to let it use at least one high frequency radio to keep open communication lines with the outside world but has not received any response yet, Ms Bunker said.