Thousands of Syrian troops have pulled out of a strategic plateau in the north western province of Idlib headed for Aleppo, which is under opposition threat, according to activists.
It is claimed goverment fighter jets are targeting Aleppo - Syria's second city - as opposition fighters attempt to seize control of the commercial hub.
In a significant escalation of the conflict, the regime has reportedly unleashed warplanes and helicopter gunships to "drop bombs" in a bid to regain control of neighbourhoods in Aleppo and the capital Damascus.
Residents and local activists say Syrian troops and President Bashar al Assad's intelligence services are battling opposition fighters around Aleppo's Old City.
"The rebels have moved to try and liberate downtown after taking over the neighbourhoods of al Sakhour, Masaken Hanano, Tariq al Bab, al Sheikh Najajr and al Ard al Hamra in the east and Saladin in the west," an opposition activist in Majed al Nour said.
Opposition activist Mohammed Saeed has estimated the rebels are holding large chunks of the city and the government has responded with attack helicopters - key to its retaking of Damascus over the last few days.
Circling fighter jets have also been breaking the sound barrier overhead in an apparent attempt to cow the fighters.
"It's like a real war zone over here, there are street battles over large parts of the city," Saeed told reporters.
"Aleppo has joined Homs and Hama and other revolutionary cities."
In a village north west of Hama, Syrian troops killed up to 30 worshippers as they entered a mosque to attend Ramadan evening prayers, according to opposition activists.
"Troops and shabbiha (militia loyal to Mr Assad) left the roadblock on the edge of Shariaa and crossed the main road and began firing automatic rifles at the worshippers as they were entering the mosque," said activist Jamil al Hamwi.
And helicopter gunships have apparently targeted fighters in northern parts of the capital Damascus, while tanks moved in to force rebels out of parts of the city they had claimed last week.
There has also been fresh violence in Homs where shelling has been reported in parts of the city.
Sky News Middle East Correspondent Emma Hurd said earlier: "The fighting is continuing around Aleppo where we have these reports now that fighter planes are in the skies and reports that they have been used to drop bombs around that area.
"Now we cannot confirm this but we certainly have seen helicopter gunships in some of the videos posted online in use around Aleppo and in use around Damascus.
Meanwhile, defected brigadier general Manaf Tlas has called for the Syrian military to denounce what he describes as crimes committed by Mr Assad's forces.
Reading a prepared statement on Saudi-based al Arabiya television, he said: "I address you... as one of the Syrian Arab Army's sons who reject the criminal behaviour of this corrupt regime.
"The honourable people in the military would not accept these crimes."
