The preliminary planning for the force is part of contacts between UN-Arab League envoy Kofi Annan and President Bashar al-Assad's government.
Annan's spokesman Ahmad Fawzi confirmed that preparations were being made by the UN's Department of Peacekeeping Operations (DPKO) without giving details.
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"Yes, DPKO is getting ready to send an advance mission," Fawzi told AFP from Geneva.
A UN official in New York, speaking on condition of anonymity, said a minimum of 250 observers would be needed if the Syrian government halted its offensive on protesters and gave its agreement for the international force.
UN peacekeeping technical experts hope to go to Damascus soon on the "advance mission" to discuss the force but the Syrian government has yet to approve the visit.
"We are waiting for the government's answer," the UN official told AFP on condition of anonymity.
"For the mission to go ahead there has to be a ceasefire, the agreement of the host country, Syria, and a Security Council mandate and for the moment there are none of those things," another senior UN official said on condition of anonymity.
But none of the elements are close to being reached. "We are very far from the idea of an observer mission. We need a peace to keep first," one senior diplomat said of the planning.
At present the plan would be for the observers to be unarmed but under the protection of Syrian government forces, the diplomat said.
The observers would mostly come from UN missions in Lebanon (UNIFIL), the Golan heights between Syria and Israel (UNDORF) and in South Sudan (UNMISS), the diplomat added.
Several diplomats stressed that the necessity of a UN Security Council resolution, along with Syria's agreement, before any force could be sent.
The 15-member council has yet to pass a full resolution on Syria. Russia and China have vetoed two European-drafted resolutions condemning the violence, saying they were unbalanced.
Security Council diplomats said though that China, at least, had indicated it would back a resolution in support of Kofi Annan's mission and any ceasefire observation.
