An attack on Syria's Idlib, a rebel-held enclave, would be a massacre and the upcoming summit in Tehran, which will be attended by Iran, Russia and Turkey, would yield positive results, Turkish President Recep Tayyip Erdogan was quoted as saying by Hurriyet Daily.
"The situation in Idlib is crucial for Turkey. A ruthless process has been going on there. ... God forbid, if this area is hailed by missiles there would be a serious massacre," Erdogan was quoted as saying.
Turkey, which has backed some rebels against Assad, is hoping for a positive outcome from the summit in Tehran which will be held later in the week.
"We will carry this issue to a positive point with the Tehran summit, which is a continuation of Astana. I hope we will be able to prevent the Syrian government's extremism in this region," Erdogan said, according to Hurriyet.
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Meanwhile, U.S. Secretary of State Mike Pompeo and Turkish Foreign Minister Mevlut Cavusoglu spoke about the Syrian conflict and the detention in Turkey of American pastor Andrew Brunson, the State Department said in a statement.
Both diplomats agreed the offensive in Idlib would be "an unacceptable, reckless escalation," State Department spokeswoman Heather Nauert said.
Russian and Syrian jets hammered a major rebel stronghold on Tuesday, a war monitor said, days before leaders of Russia, Iran and Turkey meet to discuss an expected Syrian government offensive that could spark a humanitarian disaster.
The warplanes bombarded countryside around Jisr al-Shughour on the western edge of the rebel enclave of Idlib after weeks of lull, killing 13 civilians but no fighters, according to the Syrian Observatory for Human Rights and a rebel source.
A Syrian government minister said the siege of Idlib would probably be resolved by force. "Until now, military action is more likely than reconciliations," Syrian Reconciliation Minister Ali Haidar told Russia's Arabic-language Sputnik news agency.
Pompeo and Cavusoglu also agreed to continue discussions "to resolve" Brunson's detention, she said.
The 50-year-old pastor, who is being tried on espionage and terrorism-related charges, was detained in October 2016 and arrested that December in the aftermath of a failed coup.
Relations between the two NATO allies have spiraled into a full-blown crisis over Brunson's trial, who was held for 21 months in a Turkish prison until his transfer to house arrest late in July -- a move Washington dismissed as insufficient.