Britain-based Syrian Observatory for Human Rights says heavily-bombarded Maarat al Numan "is nearly besieged," with residents of several villages scurrying for safety in the northwest of the country.
Syrian regime forces were poised on Monday to soon enter Maarat al Numan, a town of symbolic and strategic importance that is deserted after months of bombardment, in the country's last major opposition and rebel bastion.
Regime loyalists have since Friday seized around 14 towns and villages around the city, reaching its eastern outskirts, the Britain-based Syrian Observatory for Human Rights said on Monday.
They have also cut a section of the M5 highway leading north from Maarat al Numan to Idlib city, according to the Observatory and the pro-regime Al Watan newspaper.
Retaking full control of the highway is essential to the regime's efforts to rekindle a moribund economy.
The fighting has forced hundreds of thousands of civilians to flee their homes, with hundreds of vehicles on Monday packing a road leading out of the flashpoint region under heavy bombardment.
"Maarat al Numan is nearly besieged," said Observatory head Rami Abdul Rahman, explaining that regime forces were now stationed south, east and north of the city.
Maarat al Numan is a strategic prize lying on the M5 highway linking Damascus to Syria's second largest city Aleppo, the main artery coveted by the regime.
It is also the second biggest city in the beleaguered northwestern province of Idlib, the last stronghold of anti-regime forces and home to some three million people — half of them displaced by violence in other areas.
Abdul Rahman said regime forces and loyalists were now pushing from the west and northwest in a bid to tighten the noose around the opposition and rebel holdout.
An AFP correspondent in the region said regime forces were also trying to reach the city's southwestern edges to prevent rebels and militants from falling back.
Idlib and nearby areas of Hama, Aleppo, and Latakia provinces are dominated by the Hayat Tahrir al Sham (or HTS) militant group, led by members of the country's former Al Qaeda branch.
In recent months, the regime of Bashar al Assad has chipped away territory under militants' control in the four provinces, despite several ceasefire agreements.
Assad has repeatedly vowed to reassert control over the whole of Syria.
An AFP correspondent said Maarat al Numan had become a ghost town, but the Observatory maintained that some civilians had remained in the area despite the escalation.
Fearing further regime advances, residents of several towns and villages located north of Maarat al Numan, have started to flee, according to the Observatory and an AFP correspondent.
Pick-up trucks carrying entire families from the town of Saraqib and the Jabal al Zawiya region packed a road leading north towards the border with Turkey, said an AFP correspondent.
The vehicles were crammed with mattresses, clothes and household appliances, many of them belonging to families who had previously fled Maarat al Numan.
'Towards the unknown'
Sitting in the front seat of a pick-up truck, Umm Hussein used a tissue to dab tears off her face, her watery blue eyes glistening from behind the car window.
"I have left behind my relatives and even my daughter ... who is on the verge of giving birth," said the mother of seven from the Jabal al Zawiya region, her voice breaking as she spoke.
"I don't know what's going to happen to them, I don't know what's going to happen to my daughter," she added, saying there weren't enough vehicles to get them out.
Despite the heavy movement of civilians, regime-ally Russia on Monday pounded areas around the key exit route, killing two civilians, the Observatory said.
Plumes of white smoke were visible to motorists trapped in the congestion on the highway, an AFP correspondent said.
"We are heading towards the unknown," said one elderly woman, three little children sitting beside her in a pick-up truck.
Maarat al Numan was an early hub of support for what started as demonstrations against Assad's rule in 2011.
Assad's forces, which are also battling HTS militants in western Aleppo province, are backed on both fronts by the Syrian regime and Russian air strikes.
Since December 1, some 358,000 Syrians have been displaced from their homes in Idlib, the vast majority of them women and children, according to the United Nations.
The UN says an additional 38,000 people fled violence in neighbouring western Aleppo between January 15 and 19.
A ceasefire announced by Moscow earlier this month was supposed to protect Idlib from further attacks, but it never took hold.
Aid agencies and relief groups have warned that further violence could fuel what may potentially become the largest wave of displacement seen during Syria's nine-year-old civil war that has killed more than 400,000 people and displaced millions of others.
Syrian regime forces now control around 70 percent of the country.