"The killing of Soleimani... was an adventurist step that will increase tensions throughout the region," news agencies RIA Novosti and TASS quoted the foreign ministry as saying.

"Soleimani served the cause of protecting Iran's national interests with devotion. We express our sincere condolences to the Iranian people."

Russia and Iran are key allies in the Middle East, with the militaries of both countries backing President Bashar al-Assad's regime in Syria.

Moscow is also one of the world powers that negotiated the landmark Iran nuclear deal that Washington withdrew from in 2018, leading to a surge in tensions.

Konstantin Kosachev, the head of the foreign affairs committee of the Russian parliament's upper house, said Soleimani's killing was a mistake that would hit back at Washington.

"Retaliatory strikes will certainly follow," he said in a post on his Facebook page, adding that Israel was likely also worried.

He said the killing marked the end of any chance to salvage the nuclear deal.

"The last hopes of resolving the Iranian nuclear problem have been 'bombed'," he wrote.