The Security Council will let the UN military observer mission's mandate in Syria expire Sunday and will back a new civilian office there to support UN and Arab League efforts to end the country's 18-month conflict.

France's UN Ambassador Gerard Araud, the current Security Council president, said Thursday that members agreed to UN Secretary-General Ban Ki-moon's proposal for a liaison office.

Araud said the council agreed that conditions set for possibly extending the mission of the unarmed observers past Sunday were not met. He says there was no halt to the Syrian government's use of heavy weapons and no significant reduction in violence.

Russia invited UN ambassadors from key world powers that met in Geneva in June and agreed on guidelines for a political transition in Syria to meet in New York on Friday and call on all sides to end the violence.

"More than 18,000 people have been killed during the last 18 months," Ban told reporters in East Timor on Wednesday. "The Syrian people have suffered too much too long."

In a letter to the council last Friday, Ban said, "it is imperative for the United Nations to have a presence in Syria" aside from its humanitarian operation in order to support UN and Arab League efforts "in mediating and facilitating a peaceful resolution to the crisis."

The Security Council initially authorized the 300-strong 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 cease-fire 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 and insecurity, the observers have been mainly confined to their hotels since June 15, and their numbers have been cut by about two-thirds. The UN said Wednesday that 110 observers remain in Syria, mainly in Damascus. A bomb exploded Wednesday outside their hotel, wounding three people, but no observers were hurt.

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.