Iranian President Hassan Rohani attacked the United States and Israel for their roles in Syria before he boarded a plane for Turkey where he was scheduled to meet Turkish President Tayyip Erdogan and visiting Russian President Vladimir Putin.
"The Americans are against the Syrian government establishing its authority throughout the whole country and are even thinking of a breakup [of the country]," he said in a ceremony broadcast live from Tehran's Mehrabad airport.
"There's interference from Zionist forces in Syria which has increased problems. They don't respect Syrian national sovereignty. They bomb areas in Syria. They support terrorists. These are all issues which have increased Syria's problems," he said.
The leaders of the three countries are meeting in Ankara for a summit on Syria, bringing together two powers which have been Syrian President Bashar al-Assad's most forceful supporters, Iran and Russia, with one of his strongest opponents, Turkey.
Russia has boosted Assad's military capabilities in the conflict by bringing in key air support.
Critics have accused Russia of targeting civilians, a charge Moscow has denied.
The U.S. military has primarily limited its intervention in Syria to fighting Islamic State militants rather than targeting forces allied with the Syrian government.
Rohani said there was no military solution to seven years of conflict in Syria, in which 500,000 people have been killed and half the population displaced. Only dialogue could end it, he said.
Shi'ite-dominated Iran is Assad's main regional ally and has acknowledged having military advisers and volunteers from its elite Revolutionary Guards in Syria.
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Tensions ramped up between Israel and Iran in early February when anti-aircraft fire downed an Israeli F-16 warplane returning from a bombing raid on Iran-backed positions in Syria.
Israel then launched a second and more intensive air raid, hitting what it said were 12 Iranian and Syrian targets in Syria, including Syrian air defense systems.
Senior Iranian political and military officials have denied playing a role in shooting down the Israeli jet or launching a drone into Israeli territory.