United Nations – UN Secretary-General Antonio Guterres is encouraged that a ceasefire in South Sudan is largely holding and violence among signatories to September's peace agreement has decreased – but he says sporadic clashes continue indicating "the situation remains fragile".And the UN chief warns that key benchmarks haven't been achieved five months into an eight-month period preceding the political transition planned under the accord.These include "silencing the guns", reaching agreement on a future vision for the security sector, establishing transitional security arrangements, and forming a transitional government that meets agreed quotas for women and the opposition, Guterres says.Guterres stressed in a report to the UN Security Council circulated on Tuesday that after five years of civil war, the agreement between the government and key opposition groups is "the best and only option for a political solution to the conflict in South Sudan".There were high hopes that South Sudan would have peace and stability after gaining its independence from neighbouring Sudan in 2011. But it plunged into ethnic violence in December 2013, when forces loyal to President Salva Kiir, a Dinka, started battling those loyal to Riek Machar, his former vice president who is a Nuer.Fighting has killed almost 400 000 people, displaced millions and left more than 7 million – two-thirds of the population – "severely food insecure" and in need of humanitarian aid. Many peace agreements have failed, but since the September deal was signed, the previously warring parties have been trying to rebuild trust.'Some positive momentum'Guterres said "some positive momentum" has been generated by the return of senior opposition leaders to the capital of Juba but "much remains to be done".He said confidence-building among the parties to the agreement has been continuing, with military commanders engaged in local "rapprochement efforts, while local authorities and communities held peace celebrations in several locations".But he said activities to implement the peace deal have focused only on procedural issues, "and timelines to meet the political and security benchmarks of the pre-transition period have slipped".Guterres said the overall security situation improved between December and the end of February. Nonetheless, he said, "violence, including sexual violence against women and girls, abduction, attacks and ambushes of civilians, continued at alarming levels alongside intercommunal and intra-communal violence and cattle raiding" in the South Sudan states of Lakes, Jonglei and Warrap.The secretary-general proposed that the mandate of the UN peacekeeping force in South Sudan, which has about 14 900 military personnel and 1 870 police, be renewed for one year and be strengthened to allow the UN to support implementation of the peace process "in a nimble and flexible manner, including through the provision of technical assistance for peacebuilding priorities".He said he will assess requirements and recommend possible adjustments to the mandate "once a security arrangement has been reached".The Security Council is scheduled to vote on renewing the South Sudan peacekeeping mission March 14.