(CNN) -- South Sudanese armed forces admitted shooting down a United Nations helicopter in eastern South Sudan on Friday.
The Sudan People's Liberation Army said it was an accident, according to spokesman Philip Aguer.
Four Russian crew members were killed, the United Nations Mission in South Sudan said. The group reported that there were no passengers on board.
Aguer said that a craft was seen about 9 a.m. hovering near an SPLA command area. The army asked the U.N. mission whether it had sent a plane.
"The U.N. said there is no plane, they don't have (a) plane in the area," Aguer said.
The troops thought the aircraft looked to be the type that had dropped munitions to militias in the area, he said.
Aguer was unable to specify what was used to down the helicopter except that "anti-aircraft weapons" were used.
The helicopter was an MI-8, the U.N. mission said, and it went down near the settlement of Likuangole, in the state of Jonglei, in the country's east.
The U.N. mission is investigating the incident.
U.N. Secretary-General Ban Ki-Moon condemned the shooting down of the helicopter.
In a released statement, he said that the helicopter was clearly marked as being a U.N. aircraft.
Ban extended condolences to the crew members and called on the government of South Sudan to bring whoever is responsible to justice.
The U.N. Security Council similarly deplored the incident, saying it "constituted a grave violation" and jeopardized the U.N. mission there.
The helicopter had been on a reconnaissance flight about 10 a.m. local time, according to the mission. The mission launched a search and recovery effort that confirmed the crew members' deaths, it said.
Journalist Hannah McNeish contributed to this report.