Russian President Vladimir Putin on Sunday slammed the United States and Israel's killing of Iran's supreme leader Ayatollah Ali Khamenei in as a "cynical violation" of morality and law.

Tehran has stood by as one of Russia's closest allies throughout Moscow''s Ukraine offensive and the Kremlin had earlier called for restraint in the run-up to the US-Israeli strikes on Iran.

On Sunday, the Kremlin published a letter that Putin sent to Iranian counterpart Masoud Pezeshkian in which the Russian leader expressed his "deepest condolences for the assassination of" Khamenei.