UN Secretary-General Ban Ki-moon Thursday urged all parties in South Sudan to ensure protection of civilians as fresh fighting between pro-government and opposition forces intensified in the world's youngest nation.

In a statement issued by his spokesperson, Ban noted "with deep concern the reports of renewed heavy fighting in Malakal, Upper Nile State, since 18 February and the catastrophic consequences for civilian populations."

"Those responsible for crimes committed against civilians in South Sudan will be held accountable," Ban said, urging all parties to the conflict to respect International Human Rights and Humanitarian Law and ensure that civilians are protected.

The United Nations peacekeeping mission in South Sudan (UNMISS) has reported that heavy fighting broke out Tuesday between Sudan People's Liberation Army (SPLA) opposition forces and government troops near its compound in Malakal.

Both the government and the rebels accuse each other of violating the cessation of hostilities agreement signed last month between the two groups at peace negotiations in the Ethiopian capital, Addis Ababa.

In the statement, the UN chief reiterated the necessity for the parties to implement fully the ceasefire agreement, and to cooperate fully with the Intergovernmental Authority on Development (IGAD) Monitoring and Verification Mechanism, which the UN stands ready to support.

Ban also called on all parties to respect the lifesaving work and ensure unhindered freedom of movement of the UNMISS and humanitarian workers.

Over the past two months, thousands of people are believed to have been killed in armed clashes that began as a political dispute between President Salva Kiir and his former deputy president, Riek Machar.

According to UN figures, some 870,000 others have been displaced with 145,000 fleeing to neighbouring countries and 75,000 to UNMISS bases.