The United Nations special envoy for Syria says eastern Aleppo could be "totally destroyed" by the end of the year due to the "cruel, constant" military activities in the city, where some 275,000 civilians are besieged.
At a news conference Thursday, U.N. envoy Staffan de Mistura and his special adviser, Jan Egeland, also called on the rebel group al Nusra to see the destruction and devastation their actions are causing on the civilians of the country.
“One thousand of you are deciding on the destiny of 275,000 civilians," de Mistura said. He also criticized the Russians for what he said was excessive bombing of eastern Aleppo, putting the civilians of the city at risk for a comparatively small number of rebel fighters.
WATCH: De Mistura on situation in Aleppo
Hours later, Russian Defense Ministry spokesman Major General Igor Konashenkov told journalists in Moscow that any airstrike or missile strike against Syrian government-controlled territory would present a "clear threat" to Russian service personnel present in Syria.
He added that the two Russian military bases in Syria — the air base in Khmeimim and the naval base in Tartus — are now protected by S-300 and S-400 surface-to-air missile systems, adding that the range of those systems would be a "surprise" for any "unidentified" aircraft.
Konashenkov referred to the U.S.-led coalition's airstrikes on Syrian army positions near Deir el-Zour last month that killed 60 Syrian soldiers, which Washington said was meant to hit Islamic State positions. "[We] have taken all the necessary measures to prevent any such mistakes with regard to Russian servicemen and military facilities in Syria," he said.
Konashenkov also cited what he said were U.S. media reports concerning "discussion in the White House of the possibility of missile and airstrikes on Syrian troop positions."
FILE -- A view toward the Turkish border from Kinsibba, Syria.
IS claims Syria border bomb blast
Also Thursday, the Islamic State group claimed responsibility for a bomb blast that killed at least 20 people, most of whom were Turkish-backed opposition fighters along Syria’s northwestern border with Turkey.
A suicide car bomber targeted a convoy of rebel troops near the border crossing in the village of Atmeh, just a short distance from where the rebels and the jihadist group had been fighting along another stretch of the border.
The Syrian Observatory for Human Rights, a British-based non-profit group, put the number of dead at 21.
The Islamic State group took credit for the attack in an online statement.
Those killed include a judge and another top civil judicial figure from the rebel-held eastern Aleppo, according to a rebel official.
One witness told the Reuters news agency that most of those killed were members of the Failiq al-Sham group, which has been fighting alongside Turkish soldiers to defeat the IS group.
Lisa Schlein contributed to this report from Geneva.