U.N. Secretary-General Ban Ki-moon is urging a \"flexible UN presence in Syria\" after the observer mission ends, saying the world body has a duty to assist the crisis-beset Syrian people. \"A continued UN presence in Syria that goes beyond our important humanitarian work would allow systematic and meaningful engagement with the Syrian stakeholders, inside the country,\" the UN chief said in a letter dated Friday to the 15 members of the UN Security Council. \"Furthermore, a flexible UN presence in Syria would provide the UN impartial means to assess the situation on the ground. \"The UN cannot discontinue its support\" while the crisis continues, he emphasized. \"Rather, we must adapt to the situation while pursuing our efforts.\" The observer mission mandate is set to expire August 19, after the council voted last month to extend it for a \"final\" 30 days. At the time, the council agreed that any further extensions would be considered only if there was \"very specific and sustainable progress on the level of violence, which should subside substantially, and on the use of heavy weapons.\" The mission -- originally 300 military observers and now reduced by half -- was deployed in April to oversee a peace plan, which should have begun with a ceasefire that never took hold. In mid-June, the observers suspended patrols as fighting intensified. The Security Council is scheduled to debate the future of the UN mission on Thursday, but so far there is little consensus. The United States has been especially skeptical about prolonging the observers\' mandate yet again. Syrian ally Russia is calling for an extension, saying the observers must continue monitoring the military situation. France\'s UN envoy Gerard Araud, who currently holds the council\'s rotating presidency, predicted that the observer mission would leave at the end of its mandate. \"I don\'t see a scenario, except changes on the ground, which would permit the continuation of the mission,\" he said. Ban acknowledged in his letter that the conditions set for continuing the mission, including an end to violence, \"have not been achieved.\" Nevertheless, the UN chief said he will work toward maintaining \"an effective and flexible UN presence in Syria\" to work toward ending hostilities and \"where possible and agreed, to support the Syrians in taking the steps they identify towards a negotiated and inclusive political settlement.\" The UN and the Arab League are expected to nominate a new envoy in the coming days, after Kofi Annan resigned from the post earlier this month. Diplomats at the UN say veteran Algerian diplomat Lakhdar Brahimi, a former top League official, has been tipped but has not yet given his official response. Arab foreign ministers will meet Sunday in Saudi Arabia to discuss the conflict in Syria and Annan\'s replacement. The observer mission could be replaced by a \"political liaison office, which would function to assist the new special envoy,\" noted one diplomat, speaking on condition of anonymity. The \"current level of violence makes it difficult to maintain observer\" patrols,\" the diplomat added.