US State Secretary John Kerry on Saturday called on the South Sudanese government and opposition forces to stop military offensives and adhere to a ceasefire deal reached in January.

Kerry spoke with South Sudanese President Salva Kiir to express "grave concern" about the ongoing conflict in South Sudan, including recent violence in the troubled young country and the deliberate targeting of civilians by armed groups on both sides of the conflict, said a statement issued by the US State Department.

Over the past two months, thousands of people are believed to have been killed by fighting that began in mid-December as a political dispute between Kiir and his former deputy president, Riek Machar.

In the telephone talk, Kerry and Kiir expressed their support for the peace process led by Inter-Governmental Authority on Development (IGAD), the regional mediation bloc.

Kerry denounced recent attacks on bases of UN Mission in South Sudan (UNMISS) and its personnel, and encouraged Kiir to ensure full and unfettered access throughout South Sudan for UNMISS, the African Union Commission of Inquiry, and the IGAD Monitoring and Verification Mechanism, according to the statement.

In addition, Kerry welcomed the decision by the South Sudanese government to release the four senior political officials who had been in detention since December.