The move came as the UN humanitarian coordinator in the country said the death toll from the past 10 days of violence was likely to be in the thousands, rather than the hundreds previously estimated.
Toby Lanzer said: "I think it's undeniable at this stage that there must have been thousands of people who have lost their lives."
He was speaking from a UN compound in the rebel-held city of Bentiu, capital of Unity state.
With some 45,000 civilians seeking protection at UN bases, the Security Council unanimously authorised a plan by UN Secretary-General Ban Ki-moon to boost the strength of the peacekeeping force in South Sudan to 12,500 troops and 1323 police.
The additional personnel will reinforce UN bases where civilians are seeking shelter. However, Mr Ban warned that "even with additional capabilities, we will not be able to protect every civilian in need in South Sudan".
Violence erupted in the capital Juba on December 15 and quickly spread, dividing the country of 10.8 million along ethnic lines.