The ceasefire brokered by Moscow and Washington broke down last week.
The Kremlin said on Monday it was gravely concerned by the situation in Syria where it said terrorists were using a ceasefire to regroup and wage offensives against government troops.
"The Kremlin views the situation as extremely complicated," Kremlin spokesman Dmitry Peskov told a conference call with reporters. "We are chiefly concerned that terrorists are using a ceasefire to regroup their forces, to replenish their arsenals, for obvious preparations for waging offensives."
The ceasefire brokered by Moscow and Washington broke down last week. The United States accused Russia on Sunday of barbarism as warplanes supporting government forces pounded Aleppo and Moscow said ending Syria's war was almost impossible.
Peskov said moderate opposition forces have not been separated from terrorists, repeating Moscow's reproach of the United States for failing to deliver on its promise to nudge the armed groups it has influence over to disengage from jihadi fighters.
"Because there has been no separation of moderates from terrorists, terrorists continue their encroachments, they continue offensives. Naturally the fight against terrorists is ongoing, it must not be stopped."
French and British foreign ministers echoed US criticism of Russia on Sunday, saying it could be guilty of war crimes.
Peskov said the tough Western condemnation might hinder any resolution to the Syria crisis and hurt bilateral relations with Russia. Moscow sees "absolutely no prospect" for holding a summit on Syria, he added.
"Regrettably...the ceasefire regime has not been effective enough to date," Peskov said.
"But nonetheless Moscow is not losing hope - and what is more important, not losing political will to spare no effort for achieving a steady process of political settlement in Syria. (But) so far it's been very hard."
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