Fighters of Syria's al-Qaeda affiliate and its allies have overrun the last major government-held city in the northwestern province of Idlib, according to the monitoring group.
The fighters launched their attack on Jisr al-Shughur on Thursday, seeking to capitalise on their capture of the provincial capital last month and open up a strategic assault route to the regime's stronghold on the Mediterranean coast, the group said on Saturday.
"Al-Nusra Front and the Islamist brigades ... took control of almost all the city," Rami Abdel Rahman, head of the Syrian Observatory for Human Rights (SOHR), told AFP news agency.
"Government forces fled. Thousands of rebel fighters are in the city. Clashes are continuing in some neighbourhoods where soldiers are attempting to break out."
According to reports from activists, the command of Syrian army and security units withdrew towards Latakia province in the early morning after fighters advanced in the neighbourhoods of the city.
They said that some soldiers are still besieged in their bases and negotiating for their surrendering.
Jisr al-Shughur is the second largest city to fall to fighters after they took control of Idlib city on March 28.
SOHR also reported that government forces executed at least 23 prisoners before withdrawing from Jisr al-Shughur.
"Military intelligence members executed 23 prisoners before their withdrawal from the area of the National Hospital in the southwest of Jisr al-Shughur," the Observatory said.
Two days ago fighter groups of Ahrar al-Sham, Jind al-Aqsa, Jaish al-Islam and Nusra Front announced the battle of Liberation of Jisr al-Shughur after the Syrian government transferred all its offices and military commands from Idlib city to Jisr al-Shughur.
The fighters said on their official Twitter account: "The mujahedeen have entered the city centre. The city has been liberated."
The Syrian government made Jisr al-Shughur its de facto provincial capital after the same coalition calling itself The Army of Conquest overran Idlib city last month.
The Syrian Observatory's Abdel Rahman said that the fighters' latest victory was even more important as it cut a key road between the government stronghold of Latakia on the coast and the central province of Hama.
The regime still controls a few areas in the east of Idlib province, including the town of Ariha, a military base in al-Mastumah and an airbase at Abu Duhur.
Al Jazeera has not been able to independently verify reports by activists' networks in Syria.
Source: Al Jazeera and agencies