The U.N.-Arab League plan presented by Annan, a former U.N. secretary-general, would see a U.N.-monitored end to fighting in Syria, a withdrawal of troops from opposition areas and access for humanitarian services. Syrian President Bashar Assad agreed to the initiative Tuesday but fighting has continued.
Russian Foreign Ministry spokesman Alexander Lukashevich said the opposition should back Annan's initiative.
"It's crucial in this context that Syrian opposition groups follow Damascus' example and clearly state their support of the U.N. Security Council-backed proposal," he was quoted by Russia's state-run news agency RIA Novosti as saying.
The plan offered by Annan doesn't include a call for Syrian President Bashar Assad to stand down.
The Syrian situation topped the agenda at anArab League meeting Thursday in Baghdad. Iraqi Prime Minister Nouri al-Maliki advised against a military option for Syria.
"Based on our experience in Iraq, the option to arm either side of the conflict will lead to a regional and international proxy war in Syria," he was quoted by the BBC as saying.
The United Nations estimates that around 9,000 people have died in Syria in conflicts pitting Syrian military forces against anti-government groups.