A U.N. cargo helicopter crashed Tuesday in South Sudan's Unity state killing three crew members and injuring another, the U.N. peacekeeping mission in South Sudan said. A U.N. official told The Associated Press that it appears the aircraft was shot down.

The U.N. mission said it had "dispatched a search and rescue team" to the area, near the northern oil town of Bentiu, one of the hardest fought over areas in the country's more than eight-month-long civil war. The Mi-8 helicopter generally carries between three and five crew members.

"The mission lost contact with the helicopter, which was on a routine cargo flight from Wau to Bentiu," the United Nations mission in South Sudan (UNMISS) said in a statement. "Investigations regarding the cause of the incident will begin as soon as possible."

A U.N. official, who insisted on anonymity because he was not authorized to speak to the press, told The Associated Press that contact with the helicopter was lost at about 3:19 p.m. and that it was apparently shot down near Bentiu.

The area has seen recent heavy battles between government and rebel forces, with the town of Bentiu badly damaged in the months of fighting.

Last year a U.N. helicopter crash-landed on its way from South Sudan to Ethiopia, injuring four crew members. In 2012, South Sudan gunmen shot down a U.N. helicopter, killing all four Russian crew on board.

U.N. cargo helicopters are vital to supplying peacekeeping bases across the impoverished nation, as well as providing food aid for civilians, with aid agencies warning of the risk of famine should fighting continue.

South Sudan has seen an increase in violence between rebels and the national army since December. Thousands of people have been killed and more than 1.8 million have fled civil war sparked by a power struggle between President Salva Kiir and his sacked deputy Riek Machar.

On Monday, the warring leaders signed a fresh cease-fire deal vowing to end more than eight months of conflict.

Over 40,000 civilians are sheltering in the U.N. camp in Bentiu alone, some of the almost 100,000 civilians in U.N. bases who fled there to escape killings and massacres.

Wire services