It has been the target in recent days of artillery bombing by the regime forces of President Bashar al-Assad, as well as Russian airstrikes.
Erdogan warned that the Assad regime was preparing to launch "a massive offensive against Idlib" where some 3 million people live and is one of the few safe havens for internally displaced Syrians.
"We consider it unacceptable when the protection of the civilian population is used as a pretext for letting terrorists avoid a strike", said Russian President Vladmir Putin at a news conference in Tehran Friday.
That has prompted an estimated 30,452 people to be displaced within Idlib and parts of adjacent Hama province between September 1 and 9, the UN's humanitarian coordination agency (OCHA) said Monday.
The Observatory described the attacks on the rebel-held areas as the "most intense" since August and said they killed four in Abdeen, west of Khan Sheikhoun town, including two children and a woman.
Nicholas Heras, a researcher at the Centre for New American Security, said the spike in Russian raids on Saturday aimed to put pressure on Turkey to agree to a deal for a regime takeover of Idlib.
In the meantime, the Syrian Army dispatched more troops and equipment to the contact lines with Tahrir al-Sham Hay'at (the Levant Liberation Board or the Al-Nusra Front) in a large region from Jam'iyat al-Zahra and al-Lairamoun industrial zone up to the small town of Kafr Hamra in Northwestern Aleppo and from al-Malah farms to the small towns of Haraytan, Hayan, Anadan and Bayanoun in Northern Aleppo.
Turkey's president on Monday urged the global community to be aware of its responsibilities in Syria as an assault on Idlib looms.
Erdogan called for a "truce", while Russia's president said Damascus "has a right and must eventually take under control all of its national territory".
"I am happy with the reports about an indefinite stay", O'Hanlon said, but added: "We also need a more realistic political strategy, explicitly and publicly, that ties everything together and lays out a vision and recognizes the reality of Assad's continued rule, at least in some of the country, at least for a while".
The opposition accuses Russian Federation and its allies of already striking at hospitals and civil defence centres to force rebels to surrender in a repeat of earlier, large-scale military offensives.
Elsewhere on Saturday, clashes between Kurdish forces and regime fighters in the divided northeastern city of Qamishli killed 18 combatants, the Kurdish forces and the Observatory said.
Kurdish security forces, known as Asayish, said in a statement that a government patrol entered the areas controlled by the Kurdish militia in Qamishli and began arresting civilians, then shot at a Kurdish checkpoint sparking the gun battle. The Asayish said seven of its members and 11 government personnel were killed.
There was no immediate comment on the clashes by the government.
On April 13, US bomber jets, warships, and an attack submarine launched strikes along with French and British fighter jets in a more sweeping attack. The U.S. still has 2,200 troops in northeast Syria.