A malnourished South Sudanese child displaced by the fighting in Malakal, cries as he is washed by a nurse at a feeding centre in Kodok, Fashoda county, in this May 28, 2014 file photo. [Photo/Agencies]

UNITED NATIONS - An outburst of heavy fighting between government and opposition forces in South Sudan represents "the most sustained hostilities between the two parties since May," a UN spokesman said here Wednesday.

Stephane Dujarric made the remarks in a daily news briefing here, citing reports received by the UN Mission in South Sudan ( UNMISS) on Tuesday from the Intergovernmental Authority on Development (IGAD), which documented clashes between the two sides in Fangak County, Jonglei State, in the country's north.

The fighting reportedly displaced approximately 4,000 civilians, Dujarric said.

"The (UN) Mission says that these hostilities represent the most sustained fighting to have taken place between the main parties since May," the spokesman said. "UNMISS continues to provide protection and logistical support for the monitoring and verification teams that are deployed in South Sudan as part of the UN Mission's mandate."

"In a separate incident, the mission has also received reports of an attack last Friday in Western Equatorial State by suspected members of the Lord's Resistance Army, in which a 13-year-old girl is reportedly missing," he said. "The UN Mission is looking into the incident."

Thousands of people are believed to have been killed and more than 870,000 others have fled their homes since fighting broke out on Dec 15 between the forces of President Salva Kiir and Former Deputy President Riek Machar. IGAD has been mediating peace talks between the rival sides.