Russian Defence Minister Sergei Shoigu has accused Western countries of failing to rein in violent rebels in Syria, and said the resumption of peace talks had been indefinitely delayed.
Shoigu said on Tuesday that rebels backed by Western governments had been attacking civilians in the government-held western Aleppo, despite a pause in Russian and Syrian air strikes.
"As a result, the prospects for the start of a negotiation process and the return to peaceful life in Syria are postponed for an indefinite period," Shoigu said.
Aleppo has been hit by some of the worst violence in Syria's long-running conflict, turning the once-bustling economic hub into a divided and bombed-out symbol of the brutal war.
At the moment, Aleppo's frontline runs through the heart of the city, dividing rebels in the east from government forces in the west.
Rebels launched an offensive last week against western Aleppo, more than a month into an operation by the army to retake the city's rebel-held eastern districts, which it had already put under siege.
Shoigu, who was addressing a meeting of Russian military officials, railed against those rebels and their backers, saying they had squandered a chance for peace talks.
"It is time for our Western colleagues to determine who they are fighting against: terrorists or Russia," Shoigu said, in remarks broadcast on Russian television.
"Maybe they have forgotten at whose hands innocent people died in Belgium, in France, in Egypt and elsewhere?"
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Listing attacks that had been carried out by Western-backed rebels inside Aleppo, he said: "Is this an opposition with which we can achieve agreements?
"In order to destroy terrorists in Syria, it is necessary to act together, and not put a spanner in the works of partners. Because the rebels exploit that in their own interests."
Shoigu said he was also surprised that some European governments had refused to allow Russian navy vessels bound for Syria to dock in their Mediterranean ports to refuel or take on supplies.
But he said those refusals had not affected the naval mission, or interfered with supplies reaching the Russian military operation in Syria.
Khaled Khoja, a top negotiator with the High Negotiations Committee (HNC) main opposition bloc rejected widespread criticism of attacks on civilians in western Aleppo, stressing that "as HNC we don't accept civilian targets".
Khoja acknowledged that there had been civilian casualties as the rebels struggle to break the siege, but blamed international backers such as the United States who had refused to provide accurate weapons. He told AFP news agency in Geneva that the rebels "are not targeting civilians, they are targeting the regime, but ... the bombs they are using are not perfect bombs".
On the other side, he said, "the Russians are targeting civilians, the regime is targeting civilians. It's uncomparable".
Separately, a Kremlin spokesman said that a temporary pause in Russian and Syrian government air strikes on Aleppo was in force for now, but could not be extended if the rebels in the city did not halt their attacks.
Speaking to reporters on a conference call, Kremlin spokesman Dmitry Peskov said the "humanitarian pause" in Russian air strikes in Aleppo had allowed civilians to flee, and made it possible for aid to be brought in.
"But all that is impossible if the terrorists continue to fire on neighbourhoods and humanitarian aid routes, to launch attacks, and continue to hide behind a (human) shield. That will not permit the continuation of the humanitarian pause," Peskov said.
All sides may be committing 'war crimes'
Meanwhile, all sides fighting over the Syrian city of Aleppo may be committing war crimes through indiscriminate attacks in civilian areas, UN human rights spokeswoman Ravina Shamdasani told a regular briefing in Geneva on Tuesday.
"All parties in Aleppo are conducting hostilities that are resulting in large numbers of civilian casualties and creating an atmosphere of terror for those who continue to live in the city," Shamdasani said.
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Over the weekend, the UN documented the deaths of more than 30 civilians, including 10 children, as well as dozens of injuries, resulting from strikes by mortars, rockets and other improvised explosive devices on western Aleppo, she said.
"The reported use of ground-based missiles, along with the use of armed vehicles loaded with explosives, used in an area containing more than one million civilian inhabitants, is completely unacceptable and may constitute a war crime," Shamdasani said.
The UN did not have enough information to attribute the attacks to specific groups, she said.
Government forces and their allies were also continuing to shell opposition-held eastern Aleppo, and the UN had documented at least 12 civilian casualties, including two children, on Saturday and Sunday, she said.
"Strikes against hospitals, schools, marketplaces, water facilities and bakeries are now commonplace and, if proven to be intentional, may amount to war crimes."
According to the UK-based Syrian Observatory for Human Rights, hundreds of civilians have been killed or wounded in eastern Aleppo in Syrian and Russian air strikes since the collapse of the latest ceasefire and the announcement by President Bashar al-Assad's government last month of a major ground offensive to retake the city.
Some 250,000 to 300,000 civilians are thought to be trapped in eastern Aleppo, with dwindling food supplies and extremely limited medical care in underground hospitals that have been hit repeatedly by air strikes.
Source: News Agencies