The UN Security Council voted on Tuesday to increase the number of United Nations peacekeepers in South Sudan to 12,500 troops as soon as possible.
The request by UN Secretary-General Ban Ki-moon was authorised unanimously.
The United Nations Mission in South Sudan has been struggling to keep people sheltering from the violence in their compounds safe. It has evacuated injured civilians and non-essential staff from Bor to the capital, Juba.
Thousands of people are now known to have been killed during the past week. The worsening violence is between groups loyal to the president and groups loyal to the former vice president, who was sacked in July after a power struggle.
Government troops have retaken control of Bor, the capital of Jonglei State, and a key town which last week fell to rebels loyal to the former vice-president.
Western powers and east African states are keen to prevent more chaos in a fragile region. They have tried to mediate between the president and his former deputy without success.
The discovery of several mass graves has fueled fears that ethnic bloodletting will tear the world’s newest state apart.