The United Nations Wednesday confirmed that South Sudan government forces launched on a stronghold and headquarters of rebels affiliated to exiled leader Riek Machar.

The head of the UN Mission in South Sudan (UNMISS) David Shearer, described “heavy fighting” and an “extremely worrying” situation in the area around the town of Pagak in the Upper Nile region in recent days, forcing civilians and aid workers to flee.

Speaking in the capital, Juba, Shearer said 5,000 civilians “from north of Pagak” have so far crossed the border into Ethiopia to escape the government offensive “with at least 25 aid workers evacuated in recent days because of the increasing insecurity.”

South Sudan’s civil war began in December 2013 when President Salva Kiir accused his former deputy, Riek Machar, of plotting a coup.

Tens of thousands have been killed and millions forced from their homes in the years since as a series of peace deals have been abandoned. The latest UN figures say half the population — roughly six million people — will need emergency food aid this month.

Machar was forced into exile in South Africa last year but Pagak has remained a military stronghold for his loyalists and the headquarters of his rebellion.

Shearer said he was “gravely concerned” by the situation as government forces approach Maiwut, 25 kilometres (16 miles) northwest of Pagak, and called for both parties to stop the fighting.

“The hostilities should cease to allow people to go back to their homes. There is no reason for fighting in this area,” he said.

“Irrespective of which side started it, and we’ve heard differing stories about that, we do not believe that an advance on Pagak is going to serve the interests of peace.”