Heavy Syrian and Russian air strikes on rebel-held eastern areas of Aleppo killed at least 32 civilians on Saturday, the Britain-based Syrian Observatory for Human Rights said, as Damascus seeks to recapture all of the city.
"The Syrian regime and its Russian allies commit atrocities in Syria," Muwaffaq Nirabiya, vice president of the Syrian National Coalition, told reporters in Istanbul, adding what was happening in Aleppo was "a crime".
Jawad Abu Hatab, who leads the opposition-in-exile, urged the massacres to stop as he criticised worldwide inaction.
"We cannot stand the silence of the international community," he said.
He said Damascus and its Russian allies "targeted humanitarian convoys, killed civilians, used chemical weapons -- weapons that are banned by international conventions, dropped barrel bombs against the population, razed entire buildings (and) committed massacres".
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He was referring to the attack on an aid convoy in Aleppo on Monday which left 20 dead and 18 trucks destroyed. The United States has laid the blame squarely on Russia for the incident.
Intense raids continued for a second night on Friday after the regime announced the operation to retake Aleppo on Thursday, following the collapse of a truce deal negotiated between Moscow and Washington.
Nirabya dismissed the ceasefire which fell apart earlier this month, adding that "not a single day has been without bombing and air strikes on our people".
He said Russia and Damascus closed the road to any political resolution, "preferring to opt for a military strategy until the very end".
Aleppo and its surrounding countryside have come under heavy bombardment and suffered some of the bloodiest violence during the more than five-year conflict which has left 300,000 dead and millions forced to flee their homes.