French President Emmanuel Macron called on his Iranian counterpart Hassan Rouhani on Sunday to put the "necessary pressure" on the Syrian government to halt "indiscriminate" attacks on civilians in the rebel-held enclave of Eastern Ghouta.
During a telephone call between the two leaders, Macron underscored the "particular responsiblity for Iran, because of its ties to the regime, regarding the implementation of the humanitarian truce" sought by the UN, his office said.
Their talks came as a monitoring group said forces loyal to President Bashar al-Assad had seized control of over a quarter of Eastern Ghouta, on the edge of Damascus, after two weeks of devastating bombardment.
The offensive has reportedly killed more than 640 civilians and sent hundreds more fleeing, prompting growing international calls to end the bloodshed.
"The two presidents expressed their agreement to work together in the coming days along with the UN, in conjunction with the Damascus regime and the main countries involved in Syria, to secure results on the ground, supply necessary aid to civilians and implement an effective ceasefire," Macron's office said.
He and Rouhani are expected to speak again later this week.
The United Nations has called for a 30-day ceasefire in Eastern Ghouta, but so far regime-backer Russia has declared only a five-hour daily "humanitarian pause".
Macron's office also said the French leader reiterated his support of the 2015 deal to limit Iran's nuclear activities, signed by Tehran and six world powers.
However he urged Iran to give "clear responses" to worries about the country's ballistic missile programme as well as claims that its foreign policies were heightening tensions throughout the region, in particular in Lebanon.
"He emphasised our expectation of Iran's positive contribution to regional de-escalation and efforts to resolve crises in the Middle East," the presidency said.
US President Donald Trump has threatened to pull out of the 2015 deal unless more is done to rein in Iran's missile development.
French foreign minister Jean-Yves Le Drian will hold "frank discussions" with Iran's leaders during a visit on Monday, a spokesman said.
It will be the first by a top official from the three European signatories of the nuclear deal since Trump set out his ultimatum in January.