A United Nations helicopter on a routine cargo flight in South Sudan, crashed on Tuesday killing three of its four Russian crew members, a statement released by the country's U.N. mission said. The surviving crew member received treatment from the Médecins Sans Frontieres team in Bentiu, the statement added.

“I wish to convey my heartfelt condolences to the families of the deceased and wish a full and speedy recovery to the injured crew member,” Toby Lanzer, officer-in-charge of the U.N. Mission in South Sudan, said in the statement.

The Russian Mi-8 helicopter was being used by the U.N. under contract from a Russian company for a routine cargo flight from Wau in Western Bahr El Ghazal state to Bentiu, which is about 406 miles northwest of the capital Juba. The aircraft lost contact with the U.N. mission in the region at 14:28 local time and went down near Bentiu, in oil-rich Unity State where rebels and government forces are engaged in battle.

U.N. officials said they are investigating the crash but did not respond to claims made by a rebel commander in the area who said that his fighters shot down the aircraft with rocket-propelled grenades, the New York Times reported.

The crash took place one day after representatives from both sides of the conflict in South Sudan signed an agreement in Ethiopia where they promised to work toward a cease-fire and a national government that unites them. The move had been welcomed by U.N. Secretary-General Ban Ki-moon.

South Sudan, which has been torn by conflict, saw the start of another civil war in December, between government soldiers loyal to President Salva Kiir and rebel fighters loyal to former Vice President Riek Machar. The fighting, which eventually took a sectarian turn has forced the U.N. to send its peacekeeping troops to the region.