Syrian army troops have seized a pocket of territory in the northwest where they had encircled rebels and advanced on a Turkish military post on Friday, reclaiming towns they lost early in the war.
Government forces have pounded the south of Idlib province and nearby Hama with air and ground attacks this week, pressing an offensive that has prompted a new exodus. The escalation since late April has killed hundreds of people.
Friday's advance ends opposition presence in Hama, dealing yet another blow to insurgents who have suffered a string of crushing defeats across Syria. The northwest corner is all that remains in rebel hands after more than eight years of war.
Rebel officials did not respond to requests for comment.
President Bashar al-Assad turned to Idlib after shoring up his rule in most of Syria with Russia and Iran's help.
Still, the prospect of more advances is obstructed not only by Turkey's interests further north near its border but also US forces alongside Kurdish fighters in the northeast.
Turkish President Tayyip Erdogan told Russian leader Vladimir Putin on Friday that Syrian army attacks were causing a humanitarian crisis and threatening Turkey's national security.
In a phone call, Erdogan said that attacks by government forces violated a ceasefire in Idlib and damaged efforts for a solution in Syria, the Turkish presidency said.
The latest army gains have put Turkish troops in Idlib in the firing line and threaten Ankara's hopes of preventing a new wave of refugees on its border.
Many of the 500,000 people uprooted by the fighting have fled towards the Turkish border.
Under its deals with Russia, Ankara has forces stationed at a dozen military posts in the Idlib region, including one in the town of Morek which the Syrian army pushed into on Friday.
Turkish Foreign Minister Mevlut Cavusoglu said Ankara's troops were not trapped, and were staying put at that observation post out of choice - not necessity.
"We are discussing this issue with Russia and Iran," Cavusoglu told a news conference in Beirut. "We are there not because we cannot get out, but because we do not want to get out."
Ankara backs rebel forces that control swathes of territory north of Idlib near its border under its sphere of influence, and some that have a presence in Idlib.
A series of Russian-Turkish talks, including a truce deal brokered last year to set up a "demilitarized" buffer zone, have failed to end fighting in Idlib"
The Syrian army took six towns and nearby hills after imposing "a choking siege" on insurgents in the Hama countryside on Friday morning, state-run Ikhbariya TV said.
The capture expands state control of a highway running through them, which stretches from the capital Damascus to Aleppo city, it added.
"The north Hama countryside is now fully safe", the channel's correspondent said in a live broadcast from the town of Kfar Zita. The footage showed deserted streets lined with buildings pockmarked by shellfire.
He said soldiers were searching for landmines in the town, which insurgents had controlled for nearly seven years.
Moscow and Damascus say they are responding to militant attacks by the former Nusra Front, a jihadist alliance now known as Hayat Tahrir al-Sham which is the dominant force in Idlib.