Two Russian soldiers were killed by artillery fire from ISIL in Syria's eastern province of Deir Ezzor, where the army is battling jihadists, Russia's defence ministry said.
One serviceman was killed instantly and a second died from his wounds in hospital, according to a ministry statement quoted by Russian news agencies on Monday.
The soldiers were travelling with a convoy that was hit by "mortar fire from ISIL terrorists", according to the ministry.
Moscow announced earlier on Monday that it was providing support to Syrian government forces in their push to take Deir Ezzor, one of the last remaining cities controlled by the jihadists.
Russia intervened in Syria's six-year-old civil war in September 2015 on the side of president Bashar Al Assad's government, and has conducted thousands of air strikes against targets it labels as "terrorists".
It also has a troop presence on the ground and the two soldiers reported killed on Monday bring to 34 the number of Russian service personnel killed in Syria, according to defence ministry figures.
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Meanwhile, Syria’s army continues to battle ISIL on the edges of Deir Ezzor, seeking to break the siege of a government enclave and oust the jihadists from a key stronghold.
ISIL has already lost more than half of its nearby bastion of Raqqa to attacking US-backed forces, and the loss of Deir Ezzor city and the surrounding oil-rich province would leave it with only a handful of isolated outposts.
Deir Ezzor province borders Iraq, where ISIL has also been expelled from former strongholds Mosul and Tal Afar.
The jihadists hold large parts of Deir Ezzor province, and more than half the provincial capital Deir Ezzor city, the remainder of which is controlled by the government and under ISIL siege.
Syrian troops have been advancing towards Deir Ezzor city on several fronts, backed by ally Russia.
By Monday afternoon, Syrian soldiers were three kilometres from the city, the Syrian state television said in a breaking news alert.
"The Syrian army has advanced towards Deir Ezzor to break the siege on it and is now just three kilometres from the city," it said.
The Britain-based Syrian Observatory for Human Rights monitoring group said the troops and allied fighters had arrived at the Brigade 137 base on the western outskirts of the city, and were battling to reach troops besieged by IS inside.
A military source told AFP: "There have been multiple collapses of the Daesh [ISIL] line in the west of Deir Ezzor province, allowing the army to move quickly."
"The siege on the government troops will be broken within hours."
The Russian defence ministry also reported that Syrian forces were advancing on the city, backed by Russian air strikes.
"The fall of ISIL in Deir Ezzor will be a strategic defeat for the international terrorist group in Syria," it said in a statement.
ISIL seized large parts of the province, including its many oilfields, in mid-2014 as it rampaged across Syria and Iraq.
By early 2015, it had also seized parts of Deir Ezzor city and laid siege to remaining areas under government control.
The siege tightened further earlier this year, when ISIL advanced and cut the government-held parts of the city in two, with a southern section by the key military airport now divided from a northern sector.
Humanitarian crisis
An estimated 100,000 people remain in government-held parts of the city, which had a pre-war population of some 300,000.
The Observatory estimates more than 10,000 people may live in the parts of the city held by ISIL, although precise information is hard to come by.
The siege has created a humanitarian crisis in the city, with food and medical shortages and soaring prices.
The government has brought supplies in by helicopter, and the UN has periodically airdropped humanitarian aid, but the situation remains difficult for those under siege.
Conditions are also reportedly dire for civilians trapped in ISIL-held parts of the city, with activists reporting food and medical shortages as well as water and electricity cuts.
More than 330,00 people have been killed in Syria since the conflict began with anti-government protests in March 2011.
