A United Nations helicopter that crashed -- killing all four crew members aboard -- in South Sudan on Friday was shot down by the country's armed forces, according to U.N. Secretary-General Ban Ki-moon.

There were no passengers on board at the time of the crash, the U.N. Mission in South Sudan reported.

The chopper was a Russian-crewed Mi-8 helicopter attached to the U.N. Mission in South Sudan. It was shot down around 10 a.m. local time during a reconnaissance flight over the eastern state of Jonglei. The helicopter was owned by the charter company Nizhnevartovskavia and was working under U.N. contract in an area where the South Sudan army has been battling rebels, Reuters reported.

The Russian Foreign Ministry took strong exception to the incident, saying in a statement that South Sudan must "conduct a proper investigation of the incident, ... punish those responsible, and undertake all measures to guarantee nothing like this happens in the future."

The South Sudan army initially denied it had shot down the helicopter, then later said it was an accident. It claimed the chopper had been mistaken for a plane bringing supplies to rebels. They also said the incident "raises the issue of the provision of security of U.N. peacekeeping missions with new acuteness."

Both the U.N. Security Council and the secretary-general issued statements condemning the downing and calling on South Sudan "to immediately carry out an investigation and bring to account those responsible for this act." Ban also extended condolences to the families of the helicopter crew members.