Syria's most powerful ally and protector, Russia, began positioning itself Thursday for the fall of President Bashar Assad, saying for the first time that rebels might overthrow him and preparing to evacuate thousands of Russian citizens from the country.

The head of NATO echoed the Russian assessment, saying the Syrian government is near collapse following a nearly two-year conflict that has killed more than 40,000 people and threatened to ignite the Middle East. Assad appears to be running out of options, with insurgents at the gates of the capital and the country fracturing under the weight of a devastating civil war.

"An opposition victory can't be excluded, unfortunately, but it's necessary to look at the facts: There is a trend for the government to progressively lose control over an increasing part of the territory," Russian Deputy Foreign Minister Mikhail Bogdanov, Moscow's Middle East envoy, said during hearings at a Kremlin advisory body.

Still, Bogdanov gave no immediate signal that Russia would change its pro-Syria stance at the U.N. Security Council, where Moscow has shielded Damascus from world sanctions.

Russia's acknowledgment that Assad could lose the fight is an embarrassing blow to the regime, which describes the rebels as terrorists sent from abroad with no popular support.

But the rebels have made significant gains in recent weeks, seizing large swaths of territory in the north and expanding their control on the outskirts of the capital, pushing the fight closer to Assad's seat of power.

The opposition still faces enormous obstacles, however, including the fact that some of its greatest battlefield successes are by extremist groups the West does not want to see running Syria — something that could hamper international support.