UNITED NATIONS - UN Secretary-general Ban Ki-moon on Friday strongly condemned the shooting-down of a UN peacekeeping mission helicopter in South Sudan, which killed all the four Russian crew members aboard, and urged the South Sudanese government "to immediately carry out an investigation and bring to account those responsible for this act."

"The secretary-general strongly condemns the shooting down today of a clearly marked UN helicopter by the Sudan People's Liberation Army near Likuangole, in Jonglei State of South Sudan," said a statement issued here by Ban's spokesman.

"The secretary-general calls on the government of South Sudan to immediately carry out an investigation and bring to account those responsible for this act," said the statement.

The helicopter was shot down in Jonglei State in the eastern part of South Sudan, and all the four crew members aboard were killed, UN spokesman Eduardo del Buey said at a daily news briefing here earlier on Friday.

The South Sudanese military already told the UN peacekeeping mission, known as the UNMISS, that "it had shot down the helicopter," del Buey added.

South Sudan became independent from Sudan in July last year, six years after the signing of a peace agreement that ended decades of warfare between the north and the south. In the same month, the UN Security Council established UNMISS with the purpose of consolidating peace and security and helping to establish conditions for development.