In Damascus, Zarif and President Bashar al-Assad discussed "the next round of Astana talks and the importance of lasting communication between Damascus and Tehran for continued cooperation", the presidency said in a statement.
Both countries are facing a flurry of sanctions by Western nations including the United States, with Washington this month designating Iran's elite Islamic Revolutionary Guard Corps a terrorist organisation.
Syria is in the grips of a growing fuel crisis that it blames on these sanctions.
Earlier on Tuesday, Zarif met his Syrian counterpart Walid Muallem.
The September accord aimed to set up a buffer zone around Idlib, but was never fully implemented as jihadists refused to withdraw from it.
"The Astana guarantors... need to abide by the commitments linked to the Idlib file," including "disarming terrorists groups and them leaving Idlib", Muallem said, according to Al-Watan.
Regime forces have continued to bombard Idlib despite the deal, increasingly so in recent weeks.
Zarif is expected to visit Turkey after Damascus.
Tehran has provided steady political, financial and military backing to Assad throughout the war, which has killed more than 370,000 people and displaced millions since 2011.
The Syrian regime has made a military comeback with Russian military support since 2015, and now holds almost two-thirds of the country.
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