U.S. Secretary of State John Kerry has said Moscow should back, rather than hamper, American efforts to eliminate the jihadist group Islamic State.
The militant group, known as ISIS or ISIL, is a danger to Russia as well as to the U.S., Kerry said in an interview with Voice of America aired Thursday.
ISIS, which in recent months has taken control of vast and oil-rich territories in Iraq and Syria, has carried out public beheadings on two U.S. journalists and pledged to bring about a new 9/11 attack in America.
It has also personally threatened Russian President Vladimir Putin and promised to reignite jihad in the restive republic of Chechnya, which recently weathered 15 years of Islamist violence.
The United States this week announced its intentions to launch bombing strikes against ISIS forces in war-torn Syria.
But Russia on Thursday denounced the plan, saying that without a mandate from Syrian capital Damascus or the UN Security Council the bombings "would be an act of aggression, a gross violation of international law."
"I would hope that Russia will come to see that ISIL really represents a threat to them, too," Kerry said Thursday of Russia's response.
Russia has staunchly backed Syrian President Bashar Assad ever since the outbreak in 2011 of a civil war, which has seen government forces pitted against an opposition currently dominated by extremist Islamists.
About 800 Russian Islamists, including many from Chechnya, are fighting for the anti-Assad movement, according to British newspaper The Independent.
The CIA estimates ISIS' forces at between 22,000 and 31,000 fighters, an agency spokesman told the CNN news channel.