The envoys, who were speaking on condition of anonymity, said the planning for an observer mission in Syria was at a very early stage, and it was unclear whether such a mission would ever be deployed. The year-long conflict that has killed thousands of civilians shows no signs of abating.
"We are very far from a peace to keep," a senior Western diplomat said.
A spokesman for the UN Department of Peacekeeping Operations did not have an immediate comment. One diplomat said the UN peacekeeping team would arrive in Damascus in the coming days, but another said no date had been set.
The idea, which UN-Arab League envoy Kofi Annan has suggested to the Syrian government, is to have a mission of 200 to 250 observers who would be borrowed from other UN missions already deployed in the Middle East and Africa, envoys said.
Missions that could provide observers include the UN Disengagement Observer Force (UNDOF), which patrols a slice of Syrian territory to maintain a ceasefire with Israel, or the blue-helmeted peacekeeping force in Lebanon, known as UNIFIL.
Deployment of an unarmed monitoring team would require a UN Security Council resolution, which means Russia and China would have to either vote for it or abstain. Moscow and Beijing have twice vetoed resolutions condemning Syrian President Bashar al-Assad's assault on pro-democracy demonstrations.
Another senior council diplomat said that for the plan to get Russian and Chinese acceptance, Syria would have to consent to it. Several diplomats said the observers would include Arabs and non-Arabs, though the Arab League, which suspended Syria in November, would have no formal involvement.
An Arab League monitoring mission last year failed to make any difference to the crisis.
The United Nations says Assad's forces have killed 9,000 people. Damascus blames foreign-backed "terrorists" for the violence and says 3,000 soldiers and police have been killed.
Diplomats said there were many unanswered questions about a possible UN observer force for Syria, such as how to ensure full implementation of Annan's six-point peace plan, not just selected elements of it, and how to transform a ceasefire into a political process that does not simply freeze the status quo.
There would also need to be a clear mechanism dealing with violations of Annan's peace plan, which calls for an end to violence, dialogue between the opposition and government and a "political transition" that the opposition hopes will lead to Assad's ouster.
There is also a question of how such a small mission could ever provide credible monitoring violence in Syria wherever it is taking place.
Another problem with such a force is that Syrian security forces, who have been accused by the opposition of indiscriminately killing thousands of civilians, would be the ones providing protection for the monitors.
Assad said on Thursday that Syria would spare no effort to ensure the success of Annan's peace mission but warned it would not work without securing an end to foreign funding and arming of rebels opposing him.
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