Russia says it will block a Western-backed U.N. Security Council resolution that would impose tough sanctions on Syria.

Britain, France, Germany and the United States have introduced a resolution that would give Syria 10 days to stop using heavy weapons against the opposition and civilians after the resolution passes. If it fails to heed the warning, Syria could face tough new economic and diplomatic sanctions. The resolution also leaves the door open to possible military intervention.

Russia says the Council should pass an alternate resolution that simply would extend the mandate of the U.N. observer mission in Syria for another three months.

Moscow has been one of Syria's closest allies and has vetoed previous U.N. action.

Syria's Ambassador to Iraq Nawaf Fares has defected and joined the opposition to President Bashar al-Assad. Fares is urging Syrian soldiers to join him, saying there is no honor in killing one's own people.

Fares is the highest-ranking Syrian official to switch sides since the uprising against Mr. Assad's regime began 16 months ago.