South Sudan President Salva Kiir has signed a peace deal with rebels more than a week after refusing to do so, at a ceremony in the capital Juba attended by African regional leaders.
The presidents of Kenya and Uganda, and the prime minister of Ethiopia, who all helped mediate the negotiations, were at signing event on Wednesday.
Rebel leader Riek Machar signed the accord in the Ethiopian capital last week, but Kiir had said on the same day that his government needed more time to study the text.
The UN Security Council had said that it was ready to take immediate action if Kiir did not sign the agreement on Wednesday.
Al Jazeera's Anna Cavell, reporting from Juba, said the move was potentially very important and could change the lives of hundreds of thousands of people affected by the civil war.
"However, some of the top commanders have split from [rebel leader] Machar, and they say the peace deal means nothing to them, so ... it does not mean that it would end the fighting," she said.
South Sudan has been at war since December 2013, when a split within the security forces escalated into a violent rebellion led by Machar.
The ethnic nature of the violence has alarmed the international community.
A report by a UN panel of experts, made public on Tuesday, said both sides in the conflict between government forces and rebels have targeted civilians.
It also said "the intensity and brutality of the violence" since April has been the worst in an "exceedingly violent conflict".
On Tuesday, Doctors Without Borders (MSF) said two of its staff members were killed in Unity state last week.
Thousands of people have been killed. More than 1.6 million people have been displaced.
The UN said that young girls have been raped and burned alive.
Meanwhile, oil-rich South Sudan's public debt has climbed from zero at its independence in 2011 to $4.2bn as of June.
The report said the panel has started to investigate "the financing channels used by the government and the opposition to prosecute the war and into those individuals and entities who gain financially from the continuation of the conflict".
Source: Al Jazeera and agencies