Thousands of South Sudanese have been killed in over a week of violence, according to the United Nations, with reports of bodies piled in mass graves fueling fears of a worsening of ethnic bloodletting in the world’s newest state.

The U.N. Security Council approved plans on Tuesday to almost double the number of peacekeepers in the country in an effort to protect civilians.

The top U.N. humanitarian chief in the country Toby Lanzer said Tuesday there was “absolutely no doubt in my mind that we’re into the thousands” of dead, the first clear indication of the scale of the conflict engulfing South Sudan, which won independence from Sudan to much fanfare just two years ago.

Earlier, U.N. rights chief Navi Pillay said a mass grave had been found in the rebel-held town of Bentiu, while there were “reportedly at least two other mass graves” in the capital Juba.

The official toll nationwide has stood at 500 dead for days, although aid workers have said the toll was likely to be far higher.

With some 45,000 civilians seeking protection at U.N. bases, the 15-member Security Council unanimously authorized a plan by U.N. Secretary-General Ban Ki-moon to boost the strength of the peacekeeping force in South Sudan to 12,500 troops and 1,323 police.

The additional personnel will reinforce U.N. bases where civilians are seeking shelter. However Ban warned that “even with additional capabilities, we will not be able to protect every civilian in need in South Sudan.”

The grim discovery follows escalating battles between troops loyal to President Salva Kiir and those backing his rival Riek Machar, who was vice president until Kiir sacked him in July.

Violence erupted in the capital Juba on Dec. 15 and quickly spread, dividing the land-locked country of 10.8 million along the ethnic lines of Nuer and Dinka. South Sudan seceded from Sudan in 2011 under a peace agreement to end decades of war.

Western powers and east African states, keen to prevent more chaos in the fragile region, have tried to mediate between the two sides.

The fighting is affecting oil production in South Sudan, which accounts for 98 percent of government revenue. Its government said output had fallen by 45,000 barrels per day to 200,000 bpd after Unity state oilfields shut down.

From news reports