A top representative of Syrian opposition groups, former prime minister Riad Hijab, said Monday there would be no peace talks while "foreign forces" were bombing the country.
Hijab was chosen in December as general coordinator of several opposition groups which are preparing for possible talks with the government in Damascus.
"We cannot negotiate with the regime while there are foreign forces bombing the Syrian people," Hijab said after talks with French President Francois Hollande in Paris.
His visit to France comes two weeks ahead of proposed UN-brokered peace talks to seek an end to a conflict that has claimed more than a quarter of a million lives since it began nearly five years ago.
"We want to negotiate, we are serious, but for that, the conditions need to be met," Hijab added.
He mentioned a "massacre" in north-western Aleppo where he said Russian forces "bombed three schools... killing more than 35 children."
The Syrian Observatory for Human Rights, a British-based monitoring group, said 12 pupils and three adults including a teacher, were killed in a Russian air strike on a school in Aleppo.
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Hollande said the regime must stop "indiscriminate bombings and policies aimed at starving entire villages in flagrant violation of international law" if it was serious about the talks.
He called for "immediate humanitarian measures" in areas under siege, such as the rebel-held town of Madaya where at least 28 people have reportedly starved to death since December 1 under a regime blockade.
French Foreign Minister Laurent Fabius earlier urged Damascus to end the Madaya siege and said a halt to air strikes by the regime and its Russian ally was an "absolute necessity".
Deliveries of food, medicine and blankets to Madaya began on Monday, the International Committee of the Red Cross said.
Approximately 20,000 inhabitants of Fuaa and Kafraya in northwestern Idlib province are also to receive emergency aid.
Russia, a staunch ally of Syrian President Bashar al-Assad, began a campaign of air strikes in support of the regime in late September.
Moscow says it is targeting the Islamic State group and other "terrorists" and has dismissed reports that its raids have killed hundreds of civilians as "absurd".