Syrian regime forces opened a corridor to allow civilians wanting to leave the north-west, where the military has launched a massive air and ground assault.
The move came hours after the regime took full control of the key north-western town of Khan Sheikhoun in Idlib province on Wednesday, a war monitor said.
After months of air strikes on the area by Syrian government forces and ally Russia, and days of heavy fighting against rebels, the area around the town and into northern Hama province has been surrounded, Syrian Observatory for Human Rights chief Rami Abdulrahman told AFP.
The offensive has cut off all roads out for Turkish troops in the nearby town of Morek, who were assisting some rebel groups, Mr Abdulrahman said.
Idlib is the last opposition-held area in Syria, which has been fighting a civil war since 2011.
Ten of thousands of people have fled towards the Turkish border in recent days as battles raged in parts of Idlib and Hama in the north-west, the last big rebel stronghold.
The region is home to an estimated 3 million civilians, many of whom have already fled fighting elsewhere in the country.
The UN says hundreds of people have been killed since the offensive began in late April and more than 500,000 have been uprooted since.
Most escaped deeper into the rebel bastion and towards the border, while about 30,000 have fled into government territory.
"The Syrian Arab Republic announces the opening of a humanitarian corridor in the town of Soran to enable citizens wanting to exit," a Foreign Ministry source told state news agency Sana.
But many Syrians living in rebel-controlled areas are afraid of returning regime rule.
Despite assurances from the government that they would not face reprisals, there is growing evidence that those returning have been detained and arrested, forced to pay fines and back taxes or utility bills, and men have been conscripted into the military.
Some have disappeared or been killed.
The main insurgent group in the north-west, the Al Qaeda-linked Hayat Tahrir Al Sham, pulled out of Khan Sheikhoun on Tuesday as government forces advanced amid intense bombardment and air strikes.
The Observatory said 21 rebel fighters were killed in Wednesday's clashes, along with 10 government troops.
The region was supposed to be protected by a buffer zone deal signed last September by Moscow and rebel backer Ankara, but government and Russian forces have subjected it to heavy bombardment since late April.
About 890 civilians have been killed since then, the Observatory in Britain says.
The advance raises the stakes between Syria and Turkey, whose Foreign Minister Mevlut Cavusoglu has vowed to "do whatever is necessary to ensure the security of our soldiers and observation posts".
A Turkish military convoy crossed the border into Idlib on Monday and headed south along the highway, drawing condemnation from Damascus.
Ankara claimed an air strike targeted its troops, while a Syrian pro-government newspaper said regime aircraft had hit a rebel vehicle leading them.
The Syrian regime has accused Turkey of backing "terrorists", its term for extremists and rebels.
Retaking Khan Sheikhoun has been a key government objective because the town lies on the highway connecting Damascus to second city Aleppo.
Turkish presidential spokesman Ibrahim Kalin said on Wednesday that all of Turkey's observation posts, set up under an agreement with Russia and Iran, would remain in place and support will continue to be provided to the posts.
After a cabinet meeting, Mr Kalin said Turkey's President, Recep Tayyip Erdogan, would call Russian President Vladimir Putin and US President Donald Trump in coming days to discuss developments in Syria.