According to Stephen O'Brien, United Nations' Under-Secretary-General for Humanitarian Affairs, pro-russian militants ordered UN agencies to leave rebel-held parts of in the eastern Ukraine.
Stephen O'Brien said the agencies had been told to leave by Friday, and several international non-governmental organisations (NGO) by Saturday.
The Luhansk separatists had earlier banned 10 international humanitarian agencies citing "violations".
Last year parts of the Luhansk and Donetsk regions bordering with Russia were seized by Pro-Russian rebels backed up by russian military.
Around 8,000 people have been killed since fighting began there in April 2014, a month after Russia annexed the Crimea peninsula.
Mr.O'Brien urged separatists to ensure immediate resumption of UN activities.
He noted that restrictions on humanitarian shipments already prevented delivery of around 16,000 tons of vital supplies and almost 150,000 people are not receiving monthly food distributions.