A Russian warplane was shot down on Saturday in a rebel-held zone of northern Syria.
The pilot reported that he had ejected by parachute, the Russian Defence Ministry said, adding that he was then killed in a fight on the ground with "terrorists".
Tharir al-Sham, a jihadist group spearheaded by the former Syrian branch of al Qaeda, claimed responsibility for shooting down the plane, saying that one of its fighters had scored a direct hit with a shoulder launched anti-aircraft missile.
In a statement it said: "Let the criminal invaders know that our skies are not a picnic and they will not pass through without paying a price, God willing."
The Sukhoi-25 came down in part of Idlib province that has seen heavy air strikes and fighting on the ground between Syria's government forces backed by Russia and Iran, and rebel groups opposed to President Assad.
Syrians opposed to the Assad regime see Russia as an invading force they blame for the deaths of thousands of civilians since Moscow joined the war on the side of the government in 2015.
TASS news agency quoted the Russian Defence Ministry as saying Moscow retaliated on Saturday with a strike from an undisclosed high-precision weapon that killed more than 30 militants in Idlib province.
The US State Department said it had seen reports about the downing of the plane and allegations that the United States provided missiles to groups in Syria.
"The United States has never provided MANPAD missiles to any group in Syria, and we are deeply concerned that such weapons are being used," said State Department spokeswoman Heather Nauert.
"The solution to the violence is a return to the Geneva process as soon as possible and we call on Russia to live up to its commitments in that regard."
Syria's civil war, which is now entering its eighth year, has killed hundreds of thousands of people and driven more than 11 million from their homes.