France warned Tuesday that increasing violence in Syria could threaten peace talks scheduled to resume this week in Geneva.
Regime offensives in Aleppo and eastern Ghouta have imperilled a fragile ceasefire, a French Foreign Ministry spokesman said.
"The regime and its allies could bear the responsibility for a new humanitarian drama and the failure of inter-Syrian negotiations," the spokesman said.
UN Syria envoy Staffan de Mistura said on a visit in Tehran that he plans to restart peace talks Wednesday afternoon in Geneva.
The UN mediator travelled to Iran as part of a whirlwind tour to consult with various regional powers, in an effort to gauge their support for political change in Syria.
After meeting Iranian Deputy Foreign Minister Hossein Amir-Abdollahian, de Mistura said they had discussed the fact that "a political transition is now crucially urgent."
Tehran has been backing the Syrian regime in the civil war.
"We are happy that we are approaching a political settlement of the crisis in Syria," Amir-Abdollahian was quoted as saying by state broadcaster Press TV.
However, a senior advisor to Iran's Supreme Leader Ayatollah Ali Khamenei said Sunday that the idea of an ouster of Syrian President Bashar al-Assad was a "red line" for Iran.
The Syrian opposition has insisted repeatedly that there can be no future role for al-Assad and have demanded that he step down even before free and fair elections that the UN Security Council wants to take place within the next year and a half.
Al-Assad's government, meanwhile, will Wednesday be holding polls for the country's rubber-stamp parliament in the areas under its control.
A fragile ceasefire has been in effect in Syria since late February, but jihadist groups such as al-Nusra Front and Islamic State are not covered by its terms.
The US has expressed concern over recent violence in Syria. State Department spokesman Mark Toner said in Washington that "the vast majority of violations have been on the part of the regime."