Moscow: A Sukhoi military jet crashed while trying to take off from Russia's Hmeimim base in Syria on Tuesday, killing the crew, Russian news agencies quoted a military spokesman as saying.
"The Su-24 plane rolled off the runway and broke up while accelerating for takeoff... The plane's crew did not have time to eject and died," the spokesman said.
He did not clarify how many people died, but the Su-24 usually has a crew of two.
"According to a report from the scene of the accident, the reason could be a technical failure," he added.
Russia has staged air strikes in Syria in support of the regime of President Bashar Al Assad since 2015.
On Tuesday, the defence ministry said Russian planes are currently carrying out 150 strikes per day in eastern Syria against Islamic State jihadists.
The latest casualties put the toll of Russian servicemen officially reported killed in Syria at 37.
Last month a Russian general was killed near the eastern city of Deir Al Zor, where Russian special forces are participating in the regime's ground offensive.
Russia, meanwhile, accused the US on Tuesday of pretending to fight Islamic State and of deliberately reducing its air strikes in Iraq to allow the group's militants to stream into Syria to slow the Russian-backed advance of the Syrian army.
In the latest sign of rising tensions between Moscow and Washington, the Russian Defence Ministry said in a statement that the US-led coalition had sharply reduced its air strikes in Iraq in September when Syrian forces, backed by Russian air power, had started to retake Deir Al Zor Province.
"Everyone sees that the US-led coalition is pretending to fight Islamic State, above all in Iraq, but continuing to allegedly fight Islamic State in Syria actively for some reason," said Major-General Igor Konashenkov, a spokesman for Russia's defence ministry.
The result, he said, had been that militants had moved in large numbers from Iraqi border areas to Deir Al Zor where they were trying to dig in on the left bank of the River Euphrates.
"The actions of the Pentagon and the coalition demand an explanation. Is their change of tack a desire to complicate as much as they can the Syrian army's operation, backed by the Russian air force, to take back Syrian territory to the east of the Euphrates?," asked Konashenkov.
"Or is it an artful move to drive Islamic State terrorists out of Iraq by forcing them into Syria and into the path of the Russian air force's pinpoint bombing?"
He said Syrian troops were in the midst of trying to push Islamic State out of the city of Al Mayadin, southeast of Deir Al Zor, but that IS tried daily to reinforce its ranks there with "foreign mercenaries" pouring in from Iraq.