A war monitor says "fierce clashes" between loyalist forces, militants and allied rebels were taking place one kilometre west of Khan Shaykhun in Idlib province.
Syrian regime forces entered a key town on Sunday amid heavy fighting with jihadists and their rebel allies, a war monitor said.
"Regime forces entered the town of Khan Shaykhun for the first since they lost control of it in 2014," said Rami Abdel Rahman of the Syrian Observatory for Human Rights.
Earlier, there were reports pro-regime forces were locked in heavy fighting with militants, leaving dozens of combatants dead.
The Syrian Observatory for Human Rights (SOHR) said "fierce clashes" between loyalist forces, militants and allied rebels were taking place one kilometre (0.6 miles) west of Khan Shaykhun in Idlib province.
TRT World's Sara Firth has the latest.
The latest fighting broke out overnight Saturday to Sunday and has already killed at least 45 militants and allied rebels as well as 17 members of the pro-regime forces, the Britain-based monitor said.
The town of Khan Shaykhun lies on a key highway coveted by the regime.
The road runs through Idlib, connecting regime-held Damascus with the northern city of Aleppo, which was retaken by loyalists from militants in December 2016.
Pro-regime forces are deployed around three kilometres from the road and have been advancing over the past few days in a bid to encircle Khan Shaykhun from the north and the west and seize the highway.
On Sunday they retook the village of Tel al Nar and nearby farmland northwest of Khan Shaykhun "and were moving close to the highway," Observatory head Rami Abdel Rahman said.
But their advance from the east was being slowed down due to "a ferocious resistance" from militants and allied rebels.
A buffer zone deal brokered by Russia and Turkey last year was supposed to protect the Idlib region's three million inhabitants from an all-out regime offensive, but it was never fully implemented.
US move on north Syria safe zone 'provocative' – Iran
A US agreement to set up a safe zone in northern Syria, a close ally of Iran, is "provocative and worrisome", the Iranian foreign ministry was reported to have said by the semi-official Fars news agency.
The United States and Turkey last week agreed to set up a joint operations centre for a proposed zone along Syria’s northeast border.
"Foreign Ministry spokesman Abbas Mousavi ... said the recent announcements and agreements by American officials about creating a safe zone in northern Syria are provocative and worrisome," Fars reported.
Over 860 civilians killed since April – SOHR
Regime and Russian air strikes and shelling since late April have killed more than 860 civilians, according to the Observatory, which relies on sources inside Syria for its information.
On Sunday air strikes by the Syrian regime and its ally Russia killed two people, including a child, in the south of Idlib, the Observatory said.
More than 1,400 militants and over 1,200 pro-regime forces have been killed since April, according to the monitor.
The violence has displaced more than 400,000 people, the United Nations says.
Khan Shaykhun attack
Khan Shaykhun was hit by a chemical attack that killed more than 80 people in April 2017, attributed to the Syrian regime by the UN and international experts.
In response, US President Donald Trump ordered strikes on the regime's key Shayrat airbase.
Now almost emptied of inhabitants, Khan Shaykhun sheltered almost 100,000 people before the start of the current military escalation, the majority displaced from Hama province.
"Many of these people have been displaced up to five times," the UN's regional spokesman for the Syria crisis, David Swanson, told AFP on Saturday.
Syria's conflict has killed more than 370,000 people and displaced millions at home and abroad since starting with the repression of anti-regime protests in 2011.