Accusations were flying back and forth on Wednesday, after talks aimed at securing peace in Syria were suspended.
"I have indicated from the first day I won't talk for the sake of talking," UN special envoy Staffan de Mistura was quoted by AFP as having said in Geneva after failing over several days to get peace negotiations off the ground.
"I therefore have taken the decision to bring a temporary pause (until February 25). It is not the end or the failure of the talks," he stressed.
The announcement came as Syrian troops, helped by days of Russian air sorties, cut the last supply route linking rebels in Aleppo to the Turkish border, in a major blow to the opposition.
Aleppo, Syria's pre-war commercial capital, has been divided between loyalists in the west and rebels in the east since fighting erupted in the northern city in mid-2012.
Syrian President Bashar Al-Assad's forces backed by Lebanon's Hezbollah and other militias encircled Aleppo from the west, south and east, and have advanced from the north since last week.
In addition to losing their lifeline from Turkey, which supports Syrian rebels, the opposition forces are now threatened in parts of Aleppo that they have held for three years.
On Wednesday, the army broke a three-year rebel siege of two government-held villages and took control of parts of the supply route, the Syrian military source told AFP.
"The regime forces have done in three days in Aleppo what they had failed to do in three years, thanks mainly to Russian support," said Rami Abdel Rahman of British-based monitoring group the Syrian Observatory for Human Rights.
The regime's new success on the ground helped undermine de Mistura's efforts in recent days to coax the warring sides into six months of indirect peace talks, set out in a November roadmap proposed by outside powers.
Following the announcement of the suspension of talks, both the United States and France appeared to blame Russia and its backing of Assad for the failure to make progress.
The State Department said that the Russian air strikes were harming attempts to secure peace.
"It is difficult in the extreme to see how strikes against civilian targets contribute in any way to the peace process now being explored," State Department spokesman John Kirby said, according to AFP.
French Foreign Minister Laurent Fabius agreed, saying that "dramatic" events on the ground meant the talks had "little sense".
Meanwhile, Riad Hijab, chief coordinator of the opposition umbrella group High Negotiations Committee (HNC), said late Wednesday that the group "will not return until the humanitarian demands are met or (we) see something on the ground."
He said that the Russian-backed advances on the ground showed that the negotiation was not interested in genuine peace talks.
"The whole world sees who is making the negotiations fail. Who is bombing civilians and starving people to death," Hijab told reporters.
Bashar al-Jaafari, chief negotiator for the Syrian government, countered by blaming the suspension on opposition "preconditions" and said de Mistura announced the break only because the HNC was about to leave.
"Since its arrival... (the HNC) refused to take part in any serious talks with the special envoy," state news agency SANA quoted Jaafari as saying.
AFP contributed to this report.