(New throughout, adds comments by U.N. secretary-general, South Sudan envoy)
UNITED NATIONS, Dec 24 (Reuters) - The U.N. Security Council approved plans on Tuesday to almost double the number of U.N. peacekeepers in South Sudan as soon as possible to protect civilians from worsening violence that has pushed the world's newest state to the verge of civil war.
The 15-member council unanimously authorized a request by U.N. Secretary-General Ban Ki-moon to boost the strength of the U.N. mission in South Sudan to 12,500 troops and 1,323 police, from its previous mandate of 7,000 troops and 900 police.
Violence erupted in South Sudan's capital Juba on Dec. 15 and has spread to oil-producing regions and beyond, dividing the two-year-old land-locked country along ethnic lines. Some 45,000 civilians were seeking protection at U.N. bases.
The additional troops and police will help reinforce the U.N. bases in a bid to protect the civilians sheltering there. In the past week two U.N. peacekeepers and an International Labor Organization employee were killed, and on Tuesday three U.N. personnel were injured in Bor in Jonglei state, Ban said.
"Even with additional capabilities, we will not be able to protect every civilian in need in South Sudan," Ban told the council after the vote. "Whatever the differences, nothing can justify the violence that has engulfed their young nation."
South Sudan, with a population of 10.8 million, seceded from Sudan in 2011 under a peace deal to end decades of war in what was formerly Africa's biggest state.
Ban has said the additional 5,500 peacekeeping troops and 423 police would be drawn from nearby U.N. and African Union missions in Democratic Republic of Congo, Ivory Coast, Liberia, and the Sudanese regions of Darfur and Abyei.
South Sudan's U.N. envoy Francis Deng told the Security Council his country does "not want to fall back into the abyss of war from which they have suffered for over half a century."
"The government of South Sudan is doing as much as it can, under very difficult circumstances, to restore calm and stability to the affected areas in the country," Deng said.
Ban told the council that five infantry battalions, three attack helicopters, three utility helicopters, one C-130 military transport aircraft and three police units were needed to bolster the U.N. mission in South Sudan, known as UNMISS.
The resolution adopted by the Security Council asked Ban to initially report back in 15 days on the situation in South Sudan and then every 30 days. The U.N. mission currently has some 6,700 troops and 670 police on the ground.
Western powers and east African states, keen to prevent more chaos in a fragile region, have tried to mediate between President Salva Kiir, a Dinka, and rebel leader Riek Machar, a Nuer, who was vice president until Kiir sacked him in July.
The Security Council called "for an immediate cessation of hostilities and the immediate opening of a dialogue."
It also condemned "fighting and targeted violence against civilians and specific ethnic and other communities occurring across the country that have resulted in hundreds of deaths and casualties and tens of thousands of internally displaced."
U.N. High Commissioner for Human Rights Navi Pillay said earlier on Tuesday that a mass grave believed to contain the bodies of 75 ethnic Dinka soldiers had been discovered in the rebel-held city of Bentiu, capital of Unity state.
The Security Council condemned "reported human rights violations and abuses by all parties, including armed groups and national security forces, and emphasized that those responsible for violations of international humanitarian law and international human rights law must be held accountable."
Ban simply said: "They should know the world is watching."
(Reporting by Michelle Nichols; Editing by Toni Reinhold and David Gregorio)