The UN Security Council is almost doubling the number of peacekeepers to 12,500 in the world’s newest state.
US Secretary of State John Kerry urged both President Salva Kiir and rebel leader Riek Machar to end hostilities and begin mediated political talks.
The fighting has exposed ethnic divisions in South Sudan.
Mr Kiir is an ethnic Dinka, while Mr Machar represents the Nuer tribe.
The violence erupted on 15 December when Mr Kiir accused Mr Machar, who was vice-president until his sacking in July, of plotting a coup.
Mr Machar denies trying to seize power.
In other developments on Tuesday:
- the UN said it had reports of at least three mass graves
- Mr Kiir claimed his forces had recaptured the key town of Bor; the rebels were believed to still be in control of the town of Bentiu
- the violence has caused oil production to fall by 45,000 barrels a day, Petroleum Minister Stephen Dhieu Dau said
UN Secretary General Ban Ki-moon: “The world is watching” South Sudan
An additional 5,500 UN peacekeepers are now preparing to deploy in South Sudan, following a vote in the Security Council late on Tuesday.
The UN international police force in the country will also increase from 900 to 1,323.
The Security Council vote authorised the temporary transfer of troops, police and equipment from UN missions in a number of African countries.
UN Secretary General Ban Ki-moon warned that there could be “no military solution to this conflict”.
“This is a political crisis which requires a peaceful, political solution,” he added.
Meanwhile, Mr Kerry urged both sides to immediately begin political dialogue, his spokeswoman Jen Psaki said.
She said the US special envoy to South Sudan, Donald Booth, was in the capital Juba to try to bring both Mr Kiir and Mr Machar to the negotiating table.
President Kiir and Mr Machar, who represent South Sudan’s largest and second-largest ethnic groups, have both said they are willing to talk.
However, Mr Machar has said his detained political allies must first be freed, while Mr Kiir says there should be no preconditions.
In South Sudan, Cathy Howard, the deputy head of the UN office for co-ordination of humanitarian affairs, welcomed the move by the UN to reinforce its peacekeeping mission.
“This is very welcome news at a time of real crisis for South Sudan and its people. The violence is impacting primarily on civilians,” she told the BBC on Wednesday.
“We have more than 80,000 people that we know have been displaced, the majority of whom are sheltering at UN peacekeeping bases.
“But the key has to be political dialogue between the president and the opposition,” Ms Howard stressed.
She also said that large parts of Juba currently looked like “ghost-town” as people had fled to their villages or abroad.
On Tuesday, Toby Lanzer, the top UN humanitarian co-ordinator in South Sudan, told the BBC: “I think it’s undeniable at this stage that there must have been thousands of people who have lost their lives.
“When I’ve looked at the hospitals in key towns and I’ve looked at the hospitals in the capital itself, the range of injuries, this is no longer a situation where we can merely say it’s hundreds of people who’ve lost their lives.”
Mr Lanzer also said the number of people seeking shelter from the fighting was “tens of thousands if not hundreds of thousands”.
He said that the tensions between different communities in South Sudan was even evident within a UN base he had just visited where some 7,500 people are seeking protection.
Sudan suffered a 22-year civil war that left more than a million people dead before the South became independent in 2011.