The United Nations Security Council has approved plans to almost double the number of UN peacekeepers in South Sudan.

The 15-member council unanimously authorized on Tuesday a request by UN Secretary-General Ban Ki-moon to boost the strength of the UN mission in South Sudan to 12,500 troops and 1,323 police - up from its previous mandate of 7,000 troops and 900 police.

Earlier on Tuesday the top UN humanitarian chief in the country said thousands of South Sudanese have been killed in the week-long violence, giving the first clear indication of the scale of conflict engulfing the young nation.

"Absolutely no doubt in my mind that we're into the thousands" of dead, Toby Lanzer told reporters on Tuesday.

On Wednesday, the government said that South Sudanese troops were fighting to stop rebels loyal to former Vice President Riek Machar taking control of the major oil producing Upper Nile state capital, Malakal.

"There has been heavy fighting between the forces loyal to Dr Riek Machar and the government forces (in Malakal),"
presidential spokesman Ateny Wek Ateny told the Reuters news agency by phone.

Western powers and east African states, keen to prevent more chaos in a fragile region, have tried to mediate between President Salva Kiir and rebel leader Riek Machar, who was vice president until Kiir sacked him in July.

The rebels first attacked Malakal on Tuesday morning, he said. Ateny said Upper Nile oil fields, which produce about
200,000 barrels of oil per day, were far away from Malakal and under government control.

"The oil fields are safe," he said.

More deaths feared

The official death toll has stood at 500 for days, although numbers are feared to be far higher, with some estimating at least 1,000.

Hilde Johnson, head of the UN peacekeeping mission in South Sudan told Al Jazeera on Tuesday that "terrible atrocities have been committed and perpetrators will have to be held accountable."

She said the situation "will turn into a large scale humanitarian crisis if the violence does not stop."

The United Nations mission in South Sudan oni Wednesday denied a report of a mass grave that was issued by the office of a UN Commissioner for Human Rights.

The Berlin office of Navi Pillay on Tuesday said a grave of 75 bodies was found in Bentiu, Unity State. Later the office revised that figure to 34 bodies and 75 people feared missing.

The UN mission in South Sudan said on Wednesday that the report was an inflation of a "skirmish" that killed 15 people. UNMISS said it is still "deeply concerned" about extrajudicial killings and is investigating those reports.

Machar's forces were driven from the town of Bor on Tuesday by the army, but still hold Bentiu, capital of the key oil-producing state of Unity.

Fighting has spread to half of the nation's 10 states, with hundreds of thousands fleeing to the countryside, prompting warnings of an imminent humanitarian disaster.

Machar said, for the first time, on Tuesday that he was ready to accept a Kiir offer of talks, suggesting neighbouring Ethiopia as a neutral location.

Machar's promise of talks came shortly before the army stormed Bor town, which Information Minister Michael Makwei called a "gift of the government of South Sudan to the people".

Bor's capture, apparently without major resistance by the rebels, relieves some 17,000 besieged civilians who fled into the overstretched UN peacekeeping compound for protection, severely stretching limited food and supplies.

PHOTO CAPTION

South Sudan army soldiers stand next to a destroyed motorcycle near Bor Airport, 180 km (108 miles) northwest from capital Juba December 25, 2013.

Aljazeera