UNITED NATIONS - The United Nations said South Sudan's armed forces shot down a U.N. helicopter Friday, killing all four Russian crew members on board, an attack South Sudan's military spokesman blamed on rebel fighters.

Secretary-General Ban Ki-moon stood by the U.N. account, strongly condemning the shooting down "of a clearly marked U.N. helicopter by the Sudan People's Liberation Army" and calling on South Sudan's government to conduct an immediate investigation and prosecute those responsible.

Ban sent condolences to the families of the four crew members, whose names were not released, and to the Russian government.

The Security Council "strongly deplored" the shooting down of the helicopter by the SPLA, which it said "jeopardized" the operations of the U.N. peacekeeping mission in South Sudan and was "a grave violation" of the status-of-forces agreement between the United Nation and South Sudan. The council called for a swift investigation by South Sudan and the U.N. mission and urged South Sudan "to hold those responsible for the accident accountable and take all necessary measures to avoid such tragic accidents in the future."

Earlier, U.N. deputy spokesman Eduardo del Buey said the helicopter from the U.N. peacekeeping mission in South Sudan was on a reconnaissance mission when it was shot down. The mission said the helicopter carried no passengers.

"In subsequent communications between the mission and the South Sudanese Armed Forces, the SPLA told the mission that it has shot down the helicopter in the Likuangole area in Jonglei state," del Buey said.

Pibor County in Jonglei State, where the helicopter was shot down, has been the scene of recent clashes between rebel militia fighters led by David Yauyau and South Sudanese forces. A former member of the South Sudanese Army, Yauyau launched his rebellion after failing to win a parliamentary seat in the Sudanese general elections in April 2010. South Sudan accuses Sudan of arming Yauyau.