Last year, they had designated Idlib province in the northwest as a "de-escalation" zone where violence would halt in preparation for a countrywide ceasefire.
Since then, hundreds of thousands of people are believed to have been killed and millions more displaced by the conflict. "Don't let that happen!" he added.
The Idlib area includes both rebel groups and militants such as the Nusrah Front.
Meanwhile, monitors from the London-based Syrian Observatory for Human Rights said on September 4 that Russian warplanes, in support of Assad's forces, had resumed air strikes in Idlib after a 22-day break. There was no indication as of Tuesday that it was the beginning of a wider offensive in Idlib.
Idlib is home to some 3 million Syrians, according to United Nations estimates, and 70,000 fighters from multiple factions, including former al Qaeda affiliate Hayat Tahrir al-Sham. Russia, like the Syrian government, refers to all opposition Syrian forces as terrorists.
"The IDF [Israeli Defence Forces] will continue to take strong and determined action against Iran's attempts to station forces and advanced weapons systems in Syria", Prime Minister Benjamin Netanyahu said.
Dunford estimated there were 20,000 and 30,000 militants inside Idlib but said it was still unclear to him how large of an operation was being planned by the Russian-backed Syrian forces to clear them out of the enclave.
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Since Russia's entry into the war on his side in 2015, Assad and his other close allies, Iran and a group of regional Shi'ite militias it backs, have forced the rebels from a succession of bastions including Aleppo, Eastern Ghouta and Deraa.
"No doubt one needs to deal with this problem".
Israel has said it will not tolerate a permanent Iranian military presence in Syria, especially near the frontier between the two countries. "We are sure that the main goal of this summit will be the case of liberating Idlib and this is what the Syrian people are looking for", Syria's Foreign Minister Walid al-Moallem told Russia Today in an interview.
The presidents of Iran, Russia and Turkey will hold talks on Syria in Tehran on Friday.
During their meeting, Zarif and Assad "asserted that the pressures from some Western states on Syria and Tehran will not deter the two countries from continuing to defend their principles", the Syrian president's office said in a statement.
The International Crisis Group, a Brussels-based think tank, said an all-out assault on Idlib and its catastrophic consequences could still be avoided.
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.