The 15-member council unanimously authorised a plan by UN Secretary-General Ban Ki-moon's to boost the strength of the force in South Sudan to 12,500 troops and 1323 police, as some 45,000 civilians seek protection at UN bases.

The decision follows the discovery of mass graves in a rebel-held city: one grave with 14 bodies and a site nearby with 20 bodies, said UN human rights office spokeswoman Ravina Shamdasani.

International mediation efforts have so far failed to halt the clashes, which broke out in the capital Juba on December 15 and have now reached the oil fields in Benitu in Unity State, the cornerstone of the fledgling country’s economy.

The official death toll remains at 500, although observers say at least thousands if not more have died.

“Mass extrajudicial killings, the targeting of individuals on the basis of their ethnicity and arbitrary detentions have been documented in recent days,” the United Nations human rights chief Navi Pillay said in a statement on Tuesday.

"There are reportedly at least two other mass graves in Juba,” she said, believed to be in the areas of Jebel-Kujur and Newside.

Tensions in South Sudan’s governing party emerged in July when President Salva Kiir, who is from the majority Dinka group, sacked his deputy Riek Machar, who is from the second largest group, the Nuer.

Those tensions spilled over into fighting in the capital on December 15.

Now, Ms Pillay says, “there is a palpable fear among civilians of both Dinka and Nuer backgrounds that they will be killed on the basis of their ethnicity”.

“There needs to be clear statements and clear steps from all those in positions of political and military control that human rights violations will not be tolerated and those responsible will be brought to justice.”

At least 80,000 people have been internally displaced by the crisis, with many seeking refuge in UN compounds around the country.

However the total number of those forced to flee the fighting is believed to be much, much higher, as people take shelter in churches and other locations, the UN reported.

Several hundred civilians were reportedly arrested in house-to-house searches in Juba, while hundreds of members of the South Sudan National Police Service are also believed to have been arrested in police stations around the capital, the UN says.

Last week United Nations officials said 2000 armed youths had attacked one of its bases in the town of Akobo, killing at least 11 civilians who were sheltering there and three of the peacekeepers trying to protect them.

Both President Kiir and the deposed deputy president Machar, who is now essentially leading the rebel movement against the government, have indicated a willingness to negotiate, but a government official told Reuters it would not meet Mr Machar’s demands that detained opposition leaders be released.

The United States special envoy to Sudan and South Sudan, Donald Booth, met President Kiir in the capital and was granted access to 11 senior opposition politicians who “remain detained in Juba”.

“I can report that they are secure and well taken care of. These individuals communicated to me their desire – and their readiness – to play a constructive role in ending the crisis through peaceful political dialogue and national reconciliation.”

South Sudan only became a nation on July 9, 2011, after a decades-long civil war with the north left more than a million dead. It has experienced internal conflict ever since.

with Reuters, AP