"That is not realistic," Russian Ambassador Vassily Nebenzia told reporters.
"We would like to see a ceasefire, the end of the war, but the terrorists, I am not sure, are in agreement," he said.
The UN Security Council met behind closed doors to discuss the worsening humanitarian crisis in Syria as the death toll from a four-day assault on Eastern Ghouta soared past 200.
Sweden and Kuwait requested the talks after the UN humanitarian coordinator for Syria, Panos Moumtzis, called for the month-long ceasefire in order to reach civilians in need.
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French Ambassador Francois Delattre called for a strong response from the council, appealing for an immediate ceasefire and unhindered access for aid agencies.
"Eastern Ghouta is experiencing a Middle Ages-style siege. That is totally unacceptable," Delattre told reporters.
"In Syria, we are now back to the darkest period of this conflict, with the highest death toll among civilians over the last year," he deplored.
The United States demanded an end to the air strikes, with the State Department saying in Washington: "These attacks must end now."
More than 13.1 million Syrians are in need of humanitarian aid, including 6.1 million who have been displaced within the country during the nearly seven-year war.