The preliminary planning for the force is part of contacts between UN-Arab League envoy Kofi Annan and President Bashar al-Assad's government.
A UN official, 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.
Department of Peacekeeping Operations technical experts hope to go to Damascus soon to discuss the force but the Syrian government has yet to approve the visit, officials said.
"We want to send some experts to Damascus for talks on this and 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.
"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.
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A ceasefire is the only means to stop further bloodshed. Yet it would also enable Assad's butchers to have a breathing space. They will resume their crackdown on opposition as soon as the truce is over.