UNITED NATIONS – The United Nations said South Sudan’s armed forces shot down a U.N. helicopter on 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.’s account in a statement, 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 crew members’ families and to the Russian government.

The U.N. 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, known as UNMISS, said the helicopter was not carrying any passengers.

Pibor County in Jonglei State, where the helicopter was shot down, has been the scene of recent clashes between rebels and South Sudanese forces.