JUBA : South Sudanese troops battled rebels in Monday in a key oil town, the army said, as the UN warned soldiers had tried to forcefully enter a base sheltering thousands of civilians.

Thousands have been killed and half a million civilians have been forced to flee the fighting, which entered its sixth week Monday with peace talks in Ethiopia deadlocked. The United Nations says that atrocities including war crimes are reported to have been committed by both sides.

Secretary General Ban Ki-moon warning Sunday he was “particularly disturbed” that UN staff were threatened by government troops when peacekeepers blocked soldiers from entering their base in Bor, where thousands of civilians are sheltering from weeks of conflict.

The army has denied the incident, saying that they wanted to investigate reports rebels may have thrown off their uniforms to hide in the UN base, claims the UN has in turn denied.

But Ban said the incident was “just one of a growing number of violations” of the UN accord with the government “making it increasingly difficult” for peacekeepers to implement their mandate.

Army spokesman Philip Aguer said the military was now pushing northwards against the rebels driven from Bor.

“Bor town is calm, but we are pursuing the rebel forces,” Aguer told AFP, saying that soldiers had pushed some 65 kilometres (40 miles) from the town, the strategic state capital of Jonglei state, which they wrested back off the rebels on Saturday.

The town, which has swapped hands four times in the conflict, was left devastated with corpses littering the streets and scores of buildings razed to the ground, according to an AFP reporter who visited the town on Sunday.

Civilians recounted grim stories of how the rebels gang raped and murdered sick patients in the town’s hospital.

Regional nations are trying to broker a ceasefire but have already been drawn into the brutal five-week-old conflict, with Ugandan troops battling alongside government forces loyal to President Salva Kiir.

Heavy fighting was meanwhile reported to be continuing in and around the key oil town of Malakal, capital of Unity State and one of the main battlefields since fighting erupted last month between rival forces loyal to President Kiir and his sacked deputy Riek Machar.

“We are battling in the town to regain full control of Malakal,” Aguer added. “We control the north of the city, and the rebels have the southern part.”

Both rebels and the government have claimed in recent days to control the northeastern town, which has already changed hands twice and where rebels launched a new offensive last week.

South Sudan erupted into conflict on December 15 in what Kiir called a coup attempt by Machar, whom he sacked in July. The former vice president denies the charge and accuses his ex-boss of trying to purge his rivals.

The fighting has spiralled into ethnic killings between members of Kiir’s Dinka people - the country’s largest group - and Machar’s Nuer.

Violence is rooted in decades-old grievances between former rebels turned political leaders, combined with unhealed wounds left over from the two-decades long civil war that preceded South Sudan’s independence from Khartoum in 2011.

Last week, United Nations’ top human rights envoy Ivan Simonovic, detailed reports of mass killings, sexual violence and widespread destruction.

The South Sudanese government and rebels are holding talks in neighbouring Ethiopia, but they have made little concrete progress in more than two weeks.

Talks are being mediated by the East African regional bloc IGAD, even though Uganda is a key member and the rebels have expressed concern about its neutrality.