A Russian military helicopter has been shot down in Syria, killing all five people on board.
It was the deadliest single incident for Russia's military since its air campaign began last September.
The Mi-8 transporter was brought down in northern Idlib province, an area controlled by the rebel Nusra front, Russia said.
It was returning from delivering humanitarian aid to the besieged city of Aleppo, the defence ministry said.
Three crew and two officers were on board.
An alliance of forces, including hardline jihadist factions, is the dominant power in Idlib, but it was not clear which group brought the helicopter down.
Jabhat Fateh al-Sham - formerly known as Jabhat al-Nusra - was among those active in the province, and so-called Islamic State has vowed to carry out jihad against Russian forces.
Pictures on social media, purportedly of the helicopter downing, showed burning wreckage and bodies, with armed men milling around.
Footage showed at least one body being dragged away while another could be seen apparently being trampled on.
Russia has previously, though seldom, lost aircraft since it launched operations in support of the Syrian government at the end of September 2015.
In July, two Russian pilots were killed when their helicopter was shot down east of the city of Palmyra by Islamic State.
Last November, the pilot of a Russian fighter plane was killed when the aircraft was shot down by Turkey on its border with Syria.
A Russian marine sent on a mission to rescue the pilot was also killed when his helicopter was shot down.
Russia is a key backer of Syrian President Bashar al-Assad and is supporting pro-government forces with air strikes on rebels.
Government forces cut off rebel-held eastern parts of Aleppo last month.
Russia and Syria announced the opening of what they called humanitarian corridors for civilians and rebels wanting to surrender, but few people have reportedly used them, fearing they would be targeted.
The United Nations has warned that basic supplies for the 250,000 people who live in besieged areas would last only three weeks.
Over the weekend, rebel groups south of Aleppo launched a push to try to break the siege, in what observers said was one of the biggest counter-offensives in months.