Soleimani, who led the foreign arm of the Revolutionary Guards and played a key role in fighting in Syria and Iraq, had acquired celebrity status at home and abroad.

The U.S. early on Friday killed Iranian Major-General Qassem Soleimani, head of the elite Quds Force and architect of Iran's spreading military influence in the Middle East, in an airstrike at Baghdad airport.

Russian Senator Konstantin Kosachev has called the U.S. killing of a key Iranian military commander “the worst-case scenario,” and said he believed that Iranian retribution “will not take long,” while Foreign Ministry spokeswoman Maria Zakharova said the world is now “confronted with a new reality.”

“This is very difficult news, a harbinger of new clashes between the Americans and radical Shiites in Iraq,” said Senator Kosachev, who is the chairman of the Russian Federation Council's foreign affairs committee, in an interview with the state-run RIA Novosti news agency.

“But I will be glad to be proved wrong because wars are easy to start, but very difficult to end.”

Zakharova told the Rossia 24 television channel that “this new reality entails the killing of a representative of a government of a sovereign state, an official, these actions are completely devoid of any legal basis.”

Russia's Foreign Ministry told RIA in a separate interview that the killing of Soleimani would increase tensions in the Middle East.

“We consider the killing of Soleimani as the result of an American missile strike in the vicinity of Baghdad to be a bold step that will lead to increased tension throughout the region. Soleimani was devoted to protecting Iran's national interests,” RIA quoted the Foreign Ministry as saying.

Lawmaker Leonid Slutsky, who chairs the State Duma’s International Affairs Committee, said the killing would lead to an escalation of tension.

“The Americans have crossed the red line, and this time the consequences could be severe,” he told RIA.

Senator Kosachev posted on his Facebook page that he believed the U.S. had “bombed” all hopes of controlling the Iranian nuclear program, and that Tehran could now push ahead with the creation of nuclear weapons, “even if it wasn’t planning to do so before.”

Dmitri Trenin, a prominent Russian foreign policy expert and head of the Moscow Carnegie Center, said he believed the killing of Soleimani will further heighten tensions in the Persian Gulf.

“The killing of Soleimani will not deter Iran. More likely it will further escalate the situation in the region, starting with Iraq,” Trenin said on Twitter.

Russia and Iran have been key allies of Syrian President Bashar al-Assad during the country's civil war. The two countries, together with China held joint naval drills at the end of last month in the Indian Ocean and the Gulf of Oman.

Reuters contributed reporting to this article.