Larijani had met Russian President Vladimir Putin in the Kremlin in January, at a time when US navy ships were heading towards Iran ahead of the US-Israeli air campaign launched at the end of February.

"We firmly condemn actions aimed at harming the health and, even more, the killing of the leadership of sovereign and independent Iran. We condemn such actions," Kremlin spokesman Dmitry Peskov told reporters in a daily briefing.

Moscow is a close ally of Iran and has condemned the US and Israeli attacks, which Tehran has responded to with a barrage of missile and drone strikes on US allies across the Gulf.

Last year, the two countries, both heavily sanctioned by the West, signed a broad cooperation agreement, but it stopped short of a mutual defence pact.

The version of the document made public only vaguely mentioned that Moscow and Tehran agreed to help each other counter common "security threats" and would not provide "assistance to the aggressor" if one side was attacked.

Since the outbreak of the war, Russia has sent humanitarian aid, but otherwise declined to comment publicly on what support it has offered Iran, if any.

The Kremlin last week denied a Washington Post report that Russia had passed sensitive intelligence to Iran, including the locations of US warships and aircraft in the region.

Iran emerged as one of Russia's main allies during the war in Ukraine, supplying it at the start of the conflict with drones for Moscow to fire on Ukrainian cities.