UN Chief Ban Ki-moon on Monday welcomed South Sudanese President Salva Kiir's commitment to a cessation of hostilities and readiness to engage opposition leaders in dialogue, UN spokesman Martin Nesirky told the daily press briefing. During the phone call with President Kiir, Ban also encouraged him to consider the early release of political prisoners to ease the tension. The release of the political prisoners has been a condition for Kiir's rival former Vice-President Riek Machar to enter into any dialogue. Ban also stressed the need to hold accountable those responsible for attacks on civilians. Nesirky said Ban reiterated the UN "full support" for the Intergovernmental Authority on Development's (IGAD) mediation process and called for the full cooperation of all parties in finding a peaceful solution to the current crisis. He noted that both officials also discussed plans to urgently strengthen the UN Mission in South Sudan (UNMISS) in conformity with a Security Council authorization last week. In a related matter, the head of UNMISS Hilde Johnson and the Chief of the peacekeeping department Herve Ladsous briefed the Council earlier in the day via video conference behind closed doors. Council President Ambassador Gerard Araud of France later told reporters that the situation in South Sudan is "tragic" and that it is still not clear how many people were killed since mid December when the crisis erupted between Kiir and Machar supporters. He also said that the deployment of the 5,500 troops from UN Missions in the region and authorized by the Council last week is slow because contributing countries have to agree to that, speculating that the conflict is flaring up to strengthen the winner's hand ahead of any dialogue. Ladsous estimated that the remaining troops and police force will reach the country in the next two to three weeks. The fighting erupted in the country on December 15 when government soldiers loyal to President Kiir, who belongs to the Dinka tribe, clashed with those loyal to Machar who belongs to the rival tribe Nuer, accusing the latter of staging an attempted coup because he was dismissed in July.