Russia has called on South Sudan to punish those responsible for shooting down a UN helicopter, killing its four Russian crew, and to ensure such incidents do not happen again.

A Russian Foreign Ministry statement said the "tragic occurrence" on Friday underscored the need to provide security for UN peacekeeping missions.

South Sudan became independent in July 2011 but has struggled as the country is awash with weapons after decades of civil war with Khartoum which ended with a 2005 peace agreement.