Bellicose statements by Syrian officials, along with the mobilization of pro-government forces on the periphery of rebel territory in northern Syria, have prompted speculation that a government assault to retake Idlib is imminent.
"Idlib is a complicated issue as it is the last stronghold of rebels operating against the Syrian national government, and is also home to terrorist elements", he said.
He met with Mr Assad and his foreign minister Walid al-Moallem, who is just back from a visit to Moscow.
Israel is alarmed by the the expansion of operations by Iran and the Lebanese militant group Hezbollah to support President Bashar Assad in Syria's seven-year-long civil war.
A Syrian government minister said the siege of Idlib would probably be resolved by force. Syria's war has left more than 350,000 people dead and displaced millions since it broke out with the brutal repression of an initially peaceful uprising in 2011.
During the Monday talks, the two sides underlined the need for Iran and Syria to develop their mutual cooperation in various fields, particularly in economy, in the aftermath of the resistance's victory over terrorism.
The northwestern province is densely populated, with many civilians flocking to camps near Syria's border with Turkey as they brace for an imminent attack.
But the United States president said on Twitter that "President Bashar al-Assad of Syria must not recklessly attack Idlib Province".
Iran echoed that theme.
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An attack against the Syrian province's almost 3 million people, he said, would cause "a humanitarian catastrophe".
"They are using people as human shields".
"Idlib would see a humanitarian catastrophe that exceeds the level of any seen in the Syrian conflict so far", said Lina Khatib, head of the Middle East and North Africa program at the Chatham House think tank in London. He said he had heard reports that Damascus had set a September 10 deadline for diplomacy to work before attacking.
Iran, Russia and Turkey are known as the three guarantors of a ceasefire in Syria's de-escalation zones.
In an apparent prelude to a full-scale offensive against the Idlib region, Russian Federation resumed air strikes against insurgents on Tuesday after a three-week pause. Ankara last week added Tahrir al-Sham to its list of designated terrorist groups. When he was in Russian Federation in late August, Cavusoglu said "we have to differentiate terrorists from other people", but added that it was also important to eliminate Russia's concerns.
Iranian Foreign Minister Mohammad Javad Zarif said on Monday that terrorists should be "cleaned out" of Idlib.
Hezbollah leader also hailed the achievements made by the Syrian army and its allies against USA backed takfiri (a term used for Saudi-Wahabi influenced) terrorists.
Even a staggered offensive like that would involve fighting around Turkish observation posts, potentially triggering a new escalation in an already complex war.
But a Syrian military source later rejected the claim, stating that the base was not the target of "Israeli aggression", according to state-run Sana.
But Pyongyang has since slammed Washington for its "gangster-like" demands for complete, verifiable and irreversible disarmament. Despite Trump's all-caps tweet from April - "KOREAN WAR TO END!" - that hasn't yet happened.
Del Potro - the 2009 victor of the men's singles at Flushing Meadows - breezed past Borna Coric in the final match of the day. Serena, seeded 17th, routed Venus 6-1, 6-2 in matching the most-lopsided victory in the Williams sisters' series.
That was the first career Grand Slam final for Keys, who was one of four American women in the semifinals a year ago at the U.S. Former world No. 1 Novak Djokovic will be back on Arthur Ashe in prime time, facing American Tennys Sandgren.
The victor came after 62 minutes when Olivier Ntcham swept home to find the net at the end of a swift counter attack. A fuming Gerrard told Sky Sports: 'It's definitely a foul. "I have seen it again five times and it is a foul".
The White House on Tuesday threatened military intervention if the Syrian government used chemical weapons. If it's a slaughter, the world is going to get very, very angry. "Let's f-king kill him! Let's go in.
Amazon has become the second publicly traded company to be worth US$1 trillion (NZ$1.52 trillion), hot on the heels of Apple . Amazon has briefly become the second US-listed firm to have a market value of more than $1 trillion (£779bn).
Kaia Kanepi of Estonia congratulates Serena Williams at the net after their match during the seventh day of the US Open. But having taken her first set against Williams in five career meetings, there would not be a second.
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Kavanaugh's confirmation is complicated by the sheer number of emails that exist during his years in the White House . The Senate judiciary committee was notified of the decision to withhold documents on Friday.
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