Facing an escalating civil war in Syria, the Security Council decided Thursday to end the UN military observer mission that was sent to monitor a ceasefire that never happened and back a small new liaison office that will support any future peace efforts.

Members who have been deeply divided on tackling the 18-month conflict were united behind UN Secretary General Ban Ki-moon's proposal to replace the 300 unarmed observers with a small group of military advisers and political, human rights and civil affairs experts, France's UN Ambassador Gerard Araud, the current Security Council president, announced.

The council agreed that conditions set for possibly extending the observer mission — a significant reduction in violence and an end to the Syrian government's use of heavy weapons — had not been met and its mandate would end Sunday, he said.

Araud said it was essential politically to have all 15 members on the divided council agree to approve the new liaison office, especially in light of the seriousness of the crisis which the UN says has killed at least 18,000 people.

"What is the most important is there will be a UN presence, and we hope useful UN presence," he said.

In a surprise followup, Russia's UN Ambassador Vitaly Churkin invited UN ambassadors from key nations and international organizations, who agreed on guidelines for a Syrian-led political transition in Geneva in June, to a meeting at UN headquarters Friday to press for action.

Churkin, whose country is the most important ally of Syrian President Bashar al-Assad, told reporters he wants the Geneva action group — along with Iran and Saudi Arabia, who are not members — to make "a joint or parallel appeal to all the parties of the Syrian conflict that they end violence as soon as possible by a certain point in time."

'It is clear that both sides have chosen the path of war'—Edmond Mulet, UN official

Churkin said the appeal should also urge the government and opposition to appoint representatives "to negotiate towards a political solution, and in particular towards the establishment of a transitional governing body as provided for in the Geneva document."

In a letter to the council last Friday, Ban said the conditions for extending the observer mission had not been met, but he added that the UN must maintain a presence in Syria in order to support international efforts to broker peace.

Observers confined to hotels

The Security Council initially authorized the observer mission to deploy to Syria for 90 days to monitor implementation of a six-point peace plan brokered by UN-Arab League envoy Kofi Annan. The plan was to start with a ceasefire and withdrawal of the government's heavy weapons and culminate with Syrian-led political talks.

Assad's government and opposition forces agreed to the plan, but it was never implemented.

Because of the worsening bloodshed, the observers have been mainly confined to their hotels since June 15, and their numbers have been cut by about two-thirds.

Syrian refugee girls walk outside their tent at Al Zaatri refugee camp in the Jordanian city of Mafraq, near the border with Syria. (Ali Jarekji/Reuters)

UN Assistant Secretary General for Peacekeeping Edmond Mulet told reporters after briefing the council behind closed doors that the observer mission "will come to an end at midnight Sunday."

There are now 101 observers and 72 civilian staff members in Syria, he said. The last observers will leave on Aug. 24, though they will stop work after Sunday, he added.

Mulet said discussions are underway on the new liaison office, which he said has been approved by the Syrian government and will have about 20-30 staff members.

"It is clear that both sides have chosen the path of war ... and the space for political dialogue and cessation of hostilities and mediation is very reduced at this point," Mulet said, "but that doesn't mean ... we shouldn't face that challenge of trying to open those political spaces in the future."

Frustrated at the escalating conflict and the failure of world powers on the Security Council to unite to stop the chaos, Annan announced last month that he was resigning effective Aug. 31.

Russia and China have vetoed three Western-backed Security Council resolutions that would have stepped up pressure, especially against the Syrian government, by threatening sanctions if the fighting didn't stop.

Mulet said he expected an announcement of a replacement for Annan "very soon."

Humanitarian aid requirements soar

A top UN official, meanwhile, is urging the Syrian government to adopt a more flexible approach to the delivery of humanitarian aid in the war-torn country, where a growing number of people are in need of food, water, shelter and medical care.

UN humanitarian affairs and emergency relief co-ordinator Valerie Amos told reporters that the violence in Syria "has become more intense and is too often indiscriminate."

UN relief co-ordinator Valerie Amos arrived in Syria on Tuesday at the start of a three-day regional trip to discuss humanitarian aid for Syrian civilians trapped or displaced by the fighting, a statement said. (Khaled al-Hariri/Reuters)

"Back in March, we estimated that a million people were in need of help," Amos said Thursday. "Now as many as 2.5 million are in need of assistance and we are working to update our plans and funding requirements."

She urged all parties to do more to protect civilians and said she is lobbying the government to be "more flexible in its approach to humanitarian operations."

"There are eight international NGOs (non-governmental organizations) currently operating in the country and we have the agreement of the government that they can supply humanitarian assistance," she said.

"They have put together projects which they have submitted to [the Syrian Arab Red Crescent]. Once those are agreed we can start implementation and implementation has already started in some areas. What I would like to see is the government extending that facility to other large NGOs from the international community who we know can really help to scale up the efforts significantly. I have not been able to get that agreed."

Amos, who has been in Syria since Tuesday, visited displaced families in Damascus and An Nabk. She said they were "tired and anxious" and didn't expect to be able to safely return home soon.

With files from CBC News