A rebel coalition led by Al Qaeda's Syrian affiliate edged into the last remaining government-held city in the northwest province of Idlib on Thursday, a monitoring group said.
The lightning offensive saw the Army of Conquest, or Jaish al-Fatah in Arabic, enter outer districts of Ariha within a matter of hours, the Syrian Observatory for Human Rights told AFP.
"There was heavy shelling and rocket fire, then they stormed the city," said Observatory director Rami Abdel Rahman.
"They have entered the city and are engaged in fierce clashes on its peripheries," he added.
Ariha, which was home to 40,000 people before the conflict began, is the last remaining government-held city in Idlib province which borders Turkey.
"The march of lions has begun...Let Allah liberate Ariha," the Army of Conquest's official Twitter account posted.
The coalition, which includes Al Qaeda branch Nusra Front, has won a series of victories in Idlib, including the provincial capital on March 28 and the key town of Jisr al-Shughur on April 25.
Most recently, the rebels seized the massive Al-Mastumah military base and overran a hospital complex where regime soldiers were trapped.
Many government forces retreated from these areas to Ariha, which Abdel Rahman said was heavily defended by fighters from Iran and its Lebanese Shi'ite terror proxy Hezbollah.
The Army of Conquest vowed to consolidate its control of Idlib province, where the regime still holds Ariha, the Abu Duhur military airport and a sprinkling of minority villages and military posts.