United Nations Secretary-General Ban Ki-Moon says critical resources were withheld from the South Sudan peacekeeping mission, hindering its ability to protect civilians during recent violence there.

In a speech Wednesday at an international institute in New York, Mr. Ban said the mission was denied the use of helicopters necessary to move peacekeepers through places without roads.

Mr. Ban said he was “reduced to begging” other countries and missions for substitutes. He said the incident highlights the difficulty of fulfilling the U.N. mandate to protect civilians, when even Security Council members refuse their support.

Russia, citing security concerns for its personnel, has withdrawn four of the eight helicopters it has supplied to the peacekeeping mission in South Sudan and is contemplating withdrawing the rest.

South Sudanese authorities said this week at least 50 people were killed in a revenge attack sparked by clashes between rival communities. The world's newest country has been wracked by tribal violence and fighting between the government and rebel groups.

It also is trying to cope with an influx of refugees from neighboring Sudan and disputes with Khartoum over borders and oil revenue. The tension has raised fears of war in the region.