Hundreds of thousands of people could be killed.
Iran is backing Syrian President Bashar al-Assad in his country's civil war, along with Russian Federation and Lebanese terror group Hezbollah.
"Syria's territorial integrity should be safeguarded and all tribes and groups, as one society, should start the reconstruction process, and the refugees should return to their homes", Zarif said.
Ahead of the visit, the US State Department said Jeffrey, accompanied by US special envoy for Syria Joel Rayburn, would discuss "maintaining Israel's security while countering Iran's destabilizing activity throughout the region".
He met Assad to discuss "issues on the agenda for the tripartite meeting", according to the regime presidency's account on the Telegram messaging app.
Zarif said it was necessary to consult "with our Syrian friends" ahead of the September 7 summit, according to Fars.
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France also said there could be "disastrous consequences" if Syrian forces and their allies launch an assault on the Idlib province in northwestern Syria, the last major enclave held by rebels fighting the Iran and Russia-backed Assad regime.
However, the strongest set of fighters is led by an al Qaeda-linked group, which controls most of an area that is home to more than three million people.
In their meeting Monday, Assad and Zarif also discussed what they called "western pressure" on their two countries, in apparent reference to the USA sanctions on Iran and calls for limiting Iran's role in Syria.
Syrian state media also reported the blasts at the Mazzeh airbase, on the western outskirts of Damascus, but did not mention any casualties.
According to a CNN report late last week, the us has put together an initial list of Syrian chemical weapons facilities that could be targeted with airstrikes if Syria were to launch a new chemical weapon attack.