Rebels in Syria shot down a Russian combat helicopter on Monday amid fierce fighting around Aleppo, killing all five people on board in the deadliest single incident for the Russian military since it became embroiled in Syria's civil war 10 months ago.
The helicopter downing came as the Syrian army, under Russian air cover, fought to repel a rebel attempt to break the government's siege of Aleppo, killing more than 800 militants, according to the Russian military.
The Mi-8 helicopter was shot down in Idlib province while returning to Hemeimeem air base on Syria's coast after delivering humanitarian goods to Aleppo, the Russian Defense Ministry said. It said all three crewmembers and two military officers on board died.
There was no immediate claim of responsibility for the attack.
Videos uploaded online by Syrian opposition activists showed the burning wreckage of a Russian helicopter in footage seemingly taken in the first moments after it crashed.
In one video, men, some of them armed, were shown standing near the wreckage taking cellphone photos, some cheering and shouting "Allahu Akbar," or God is great in Arabic. The body of one Russian soldier was seen being dragged by the legs while a man jumped on the half-naked body of another soldier.
The helicopter appeared to have broken up as it crashed: Its tail could be seen lying separately from the aircraft's body in flames. A rocket pod was visible amid the wreckage, standard equipment for the Mi-8, a workhorse of the Russian air force which can be used for carrying troops and cargo and attacking ground targets.
International human rights groups have repeatedly accused Russia of hitting civilians and using cluster munitions since it began its aerial campaign in Syria.
The Russian military has denied hitting civilians, and also rejected claims that its aircraft targeted moderate elements of the Syrian opposition along with its declared targets, the Islamic State group and al-Qaida's branch in Syria, the Nusra Front.
Idlib province has a strong presence of fighters from both the Nusra Front and other groups fighting Syrian President Bashar Assad's forces. The Nusra Front announced last week that it was changing its name and relinquishing ties with al-Qaida in an attempt to undermine a potential US and Russian air campaign against its fighters.
The group is part of a coalition of insurgent groups called Jaish al-Fateh, or Army of Conquest, which has captured most of Idlib.
Russia's air campaign in Syria, launched in September, has shored up Assad's regime, which was on the verge of collapse after a series of military defeats last year, and helped it gain key ground around Aleppo and elsewhere.
On Monday, Lt. Gen. Sergei Rudskoi of the Russian military's General Staff announced that the Syrian army, relying on Russian air cover, had fended off a massive militant attack intended to break the government's blockade of the rebel-held part of Aleppo.
The offensive, launched late on Sunday, involved some 5000 militants, Rudskoi said, adding that the rebels lost more than 800 fighters, as well as 14 tanks, 10 other armoured vehicles and over 60 gun trucks.
His claims could not be independently confirmed. The Britain-based Syrian Observatory for Human Rights said dozens of militants and government troops were killed in the fighting, but didn't provide exact numbers.
Syrian opposition activists said intense fighting was still ongoing in Aleppo on Monday.
Rudskoi said 324 civilians had fled the militant-controlled part of Aleppo through corridors opened by the Syrian government on Thursday. He said 82 militants had also laid down their weapons.
The UN estimates some 300,000 people are still trapped in the rebel-held section of Aleppo, with dwindling food and medical supplies. Its Syria envoy, Staffan de Mistura, warned last week that basic supplies in eastern Aleppo could run out in three weeks.
The fierce fighting around Aleppo coincided with the passing of an August 1 deadline for a UN-supported process to start a political transition in Syria. De Mistura is struggling to restart peace talks among the warring parties in Syria, even as the Syria war has increasingly been handled by diplomacy between the US and Russia.
De Mistura said last week that he aims to call a new round of talks between government and opposition envoys in Geneva "toward the end of August," but added he wants to see the outcome of steps agreed upon in Moscow last week between the United States and Russia before setting a precise date.
US Secretary of State John Kerry said the US is focusing on efforts to "get a cessation of hostilities in place that is meaningful".
"It is critical, obviously, that Russia restrain both itself and the Assad regime from conducting offensive operations, just as it is our responsibility to get the opposition to refrain from engaging in those operations," he said.
Monday's helicopter downing marks the deadliest single incident for the Russian military in the Syria campaign. It has brought the official Russian casualty toll to 19, 16 of whom were killed in enemy fire.
Last month, two Russian airmen were killed in central Homs province when their Mi-25 helicopter was shot down by what the Defense Ministry said were Islamic State fighters.
A 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. Russia also lost a helicopter in a mission to rescue the plane's second pilot, when a Russian marine was killed.