Iran's supreme leader Ayatollah Ali Khamenei on Tuesday told visiting Turkish President Recep Tayyip Erdogan that any offensive by Ankara in northern Syria would be "detrimental" to the region.
His statement came shortly before Iran's ultra-conservative President Ebrahim Raisi hosts his Russian and Turkish counterparts for talks on the Syrian conflict.
Erdogan has repeatedly threatened to launch a military offensive in northern Syria to create a "safe zone" that would drive Kurdish militants 30 kilometres (19 miles) back from the Turkish border.
On Tuesday, Khamenei told him that "this is definitely detrimental to Syria, Turkey and the region and will not fulfil the political action expected by the Syrian government".
"Terrorism must be opposed, but a military attack in Syria will also benefit terrorists," Khamenei was quoted as saying by his official website.
The Iranian leader called for the issue to be resolved through dialogue between Ankara, Damascus, Moscow and Tehran.
Khamenei, who has the final say in major state policies, also told Erdogan that Iran would offer cooperation "in the fight against terrorism".
Erdogan arrived in Tehran on Monday night, and Russian President Vladimir Putin is also expected to land in the Iranian capital later for Tuesday's trilateral meeting.
Russia, Turkey and Iran have in recent years met to discuss Syria as part of the "Astana peace process" to end more than 11 years of conflict in the Arab country.
All three are involved in Syria, with Russia and Iran supporting the Damascus regime against its opponents, and Turkey backing rebels.
Turkey has launched waves of attacks on Syria since 2016, targeting Kurdish militias as well as Islamic State group jihadists and forces loyal to Syrian President Bashar al-Assad.
Erdogan's planned military offensive would target Kurdish fighters regarded by Ankara as "terrorists".
They include the US-backed Kurdish People's Protection Units (YPG), which formed a crucial part of an international coalition against IS jihadists in Syria.
Ankara fears a strong Kurdish presence along its border with Syria will embolden the banned Kurdistan Workers' Party, which for decades has been waging an insurgency against the Turkish state that has claimed tens of thousands of lives.
Syria's government has repeatedly condemned Turkish threats to mount a new incursion.