Russia warned Western powers on Monday against any military intervention in Syria, saying the use of force without a U.N. mandate would violate international law.

Foreign Minister Sergei Lavrov said Moscow had no plans to be drawn into a military conflict over the civil war in Syria and that Washington and its allies would be repeating “past mistakes” if they intervened in Syria.

“The use of force without the approval of the United Nations Security Council is a very grave violation of international law,” Lavrov told a news conference at which he discussed accusations by rebel forces that Syrian government forces have used chemical weapons.

Russia is Syrian President Bashar al-Assad’s key ally and one of its main arms suppliers. It is also a veto-wielding member of the U.N. Security Council, where it has protected Damascus from three consecutive resolutions that were intended to put pressure on him to end violence.

For more than two years, Moscow has demanded that any change of government in Syria must be carried out via a political process in which Assad must be part, and that no talks on Damascus’ future can be predicated on Assad’s exit from power.