A top U.N. human rights official says efforts by Russia-backed separatist rebels to block the flow of humanitarian aid to people in eastern Ukraine could amount to crimes against humanity.

Ivan Simonovic, the assistant U.N. secretary-general for human rights, criticized as a "great mistake" a move by armed groups to kick out U.N. and humanitarian aid agencies in the eastern rebel-held city of Lugansk. He spoke Tuesday to reporters in Geneva after visiting Ukraine last week.

Simonovic noted that polio cases have turned up in eastern Ukraine and said 90,000 UNICEF polio vaccines are no longer likely to reach children there.

He said blocking desperately needed aid is not just illegal but could amount to "crimes against humanity."