The conflict has intensified in Syria with the government reportedly unleashing fighter jets and helicopter gunships to "drop bombs" in a bid to wrestle back control of neighbourhoods from rebels.
It is claimed the jets have targeted the northern city of Aleppo as opposition fighters attempt to seize control of Syria's second city and commercial hub amid circling warplanes.
Syrian troops and President Bashar al Assad's intelligence services are battling opposition fighters around the Old City, residents and local activists said.
"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 their 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."
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: "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.
"It would be a significant escalation if the Syrian regime has moved from just flying these aircraft over, which is quite a threatening gesture in itself, to actually using them to attack rebel positions - a sign, perhaps, that the regime is feeling more threatened now."
However, Mustapha Abdullah, who heads a group of opposition fighters called the Martyrs of Aleppo, said a lack of ammunition was becoming a problem for rebels joining the battle.
He said government forces were also shelling the countryside north of the city to disrupt rebel advances.
The opposition Free Syrian Army had said it would target the country's commercial hub after seizing the town of Azaz further north a few days ago but it likely to prove a significantly more difficult proposition to take.
The latest fighting comes amid increasing international concern over the Syrian government's threat to use chemical and biological weapons.
It has been reported that neighbouring Jordan is considering sending sepcial forces across the border "if and when the Syrian regime falls to secure its chemical and biological weapons".
According to risk consultancy Maplecroft, the "Eager Lion" military exercise held in Jordan in May with the participation of the US and 18 other countries "included a strong focus on securing chemical and biological weapons".
The US warned and France warned Syria on Monday that the use of chemical weapons would be unacceptable after foreign ministry spokesman Jihad Makdissi said they could be used against "external aggression".
Israel has said it is concerned that chemical weapons might find their way into the hands of the Lebanese Shia militia Hezbollah, an ally of Syria, or a militant Islamist group.
However, senior defence official Amos Gilad said: "At the moment, the entire non-conventional weapons system is under the full control of the regime."
Turkish Prime Minister Tayyip Erdogan has described the anti-Assad forces as "closer than ever to victory" and warned that Turkey would respond firmly to any hostility from Syria.
Amid sequential rebel attacks on the country's two largest cities and a bombing that wiped out some of his top security advisors, President Bashar al Assad reshuffled his top security posts, dismissing one general and appointing a national security council chief to replace the one killed in the attack.