A Russian helicopter with five people on board has been shot down in Syria while carrying humanitarian supplies to Aleppo, the country's Defence Ministry has said.
Three crew and two officers were returning to Khmeimim base in Idlib province when the helicopter was brought down by ground fire.
It is the deadliest single incident for the Russian military since its entrance into Syria's civil war.
"On 1 August, an Mi-8 transport helicopter has been shot down by ground fire in Idlib province after a delivery of humanitarian aid to the city of Aleppo.
"Three crew members and two officers from the Russian Reconciliation Center in Syria were on board," the ministry said in a statement.
The Kremlin said all five personnel had died.
"From what we know from information provided by the Defense Ministry, all those who were on the helicopter died," Kremlin spokesman Dmitry Peskov told journalists.
Mr Peskov said the Russians "died heroically because they tried to move the aircraft away so to minimize losses on the ground".
Images uploaded to social media purportedly show the flaming wreckage of the helicopter along with a dead body and purported Russian identity documents taken from the wreckage.
In one video, what appeared to be a rocket pod can be seen next to the wreckage. People standing nearby are seen taking photos on their phones and shouting "Allahu Akbar," or "God is great" in Arabic.
It is currently unknown which group brought the aircraft down. Idlib province has a strong presence of fighters both for the al-Qaeda branch in Syria known as Jabhat al-Nusra and rebels.
Rebels fighting for Jaysh al-Fatah (Army of Conquest) claimed to have recovered the body of the pilot on social media.
Monday's helicopter downing was the deadliest for the Russians since Moscow began carrying out air strikes in Syria in support of Assad's forces last September.
In July, two Russian airmen were killed in the central Homs province when their Mi-25 helicopter was shot down by what the defence ministry said were IS fighters.
An Mi-28N helicopter gunship crashed near Homs in April, killing both crew members, but the Russian military said there was no evidence it came under fire.
A Russian warplane was shot down by Turkey along the Syrian border in November, and one of the two pilots was shot and killed from the ground after ejecting.
Earlier on Monday, a Syrian military official claimed that government forces repelled an attack by insurgents that was an attempt to break the siege imposed on rebel-held parts of the northern city of Aleppo.
The development came a day after Syrian rebels launched the offensive to break up the government's siege of the eastern, rebel-held part of the city.
The UN estimates some 300,000 people are still trapped in the rebel section of Aleppo, with dwindling food and medical supplies. The UN's special envoy to Syria, Staffan de Mistura, warned on Friday that basic supplies in eastern Aleppo could run out in three weeks.
Opposition activists said intense fighting was still ongoing in Aleppo on Monday.
Additional reporting by agencies
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