Aleppo has become the main battleground in Syria's 16-month popular uprising as government forces intensified their firepower in response to rebel gains in Syria's largest city.
Following a major assault on the rebels in the capital Damascus, government forces are turning their attentions to Aleppo in the north where rebels claim to control 50 percent of the city.
Aleppo, Syria's principal economic center of 2.5 million, is of strategic importance to the rebels due to its position near the Turkish border, where weapons can be acquired and foreign fighters are said to be joining the opposition.
Fighting is said to be concentrated in three neighborhoods. Forces loyal to Syrian president Bashar al-Assad have allegedly used warplanes and artillery to bomb rebels armed with rifles and machine guns.
Local activist Mohammed Saeed said there were clashes overnight that stretched into a sixth day on Thursday in parts of the city of three million. On Wednesday and Thursday, he said, government forces have been relying more heavily on intense firepower from the air.
"Regime forces have been randomly shelling neighborhoods and the civilians are terrified," he told the Associated Press via Skype.
The use of warplanes in Aleppo comes after the government last week used helicopter gunships and artillery in support of overwhelming ground forces to stop an intense bid by rebels for control of the once-impregnable capital of Damascus.
The decision to employ air power came after a trusted bodyguard planted a bomb that killed four key members Assad's inner circle, leading to speculation the 46-year old president's days in power were drawing to a close.
In the days since the bombing, Assad has struck back with all his might and shown he is still a force to be reckoned with, but observers say the regime may be at the limit of its resources.
Even with overwhelming numbers of ground forces and superior technical capabilities, it took Syria's army over a week to quell the rebel assault in the capital.
"The government reinforcements have yet to arrive," Saeed said of today's fighting in Aleppo, amid reports that clashes had spread to neighborhoods close to the center of the city.
Confirming the location of rebels in Aleppo, local activist Abu Hisham told the Wall Street Journal via Skype, "There was shelling this morning on the Salaheddine and Mashhad districts." "Now it has stopped, but helicopters are buzzing overhead," he added, saying 24 people were killed in fighting in and around Aleppo on Wednesday.
Those deaths swell a rising national death toll of about 19,000 people, most of them civilians, since Assad launched his brutal crackdown in March 2012.
UN High Commissioner for Human Rights Navi Pillay went on record earlier this year saying she believed the Syrian president should be tried for crimes against humanity.
Saying the Syrian army’s use of heavy weapons against civilians in densely populated areas was a crime under international law, Pillay added that Assad was directly responsible. "Factually, there’s enough evidence pointing to the fact that many of these acts are committed by the security forces, (and) must have received the approval or the complicity at the highest level,” Pillay said in late March.
“There is no statute of limitations so people like him can go on for a very long time but one day they will have to face justice,” she added, referring to Assad. Pillay also referred to persistent allegations of the systematic kidnap, rape, torture, and summary execution of dissidents, rebels, and their families against regime forces and allied militia groups.
Meanwhile, the White House said Wednesday that the use of heavy weapons in Aleppo showed "the depth of depravity" of Assad's regime. Spokesman Jay Carney said Syrian forces were perpetrating "heinous violence" against civilians in the city.
Activists in Aleppo said they fear the worst may yet be to come, with reports the military is rushing in reinforcements, including some 100 tanks and a larger number of other military vehicles, to try to retake the city over the weekend.
"The special forces were deployed on Wednesday and Thursday on the edges of the city, and more troops will arrive to take part in a generalised counter-offensive on Friday or Saturday," a source close to the Syrian security apparatus said.
The source said rebel fighters had brought in their own reinforcements, estimating between 1,500 and 2,000 opposition fighters had arrived from outside the city to reinforce some 2,000 already fighting in Aleppo.
"They are mainly present in the southern and eastern suburbs of the city, mainly Salaheddin and nearby districts," the security source said.
The airport is currently cut off from the city, as four of the five roads leading to it are under rebel control, the source added.
Rebels also said a regime assault appeared imminent. "The army's reinforcements have arrived in Aleppo," Colonel Abdel Jabbar al-Okaidi, a spokesman for the rebel Free Syrian Army in Aleppo, told AFP via Skype.
"We expect a major offensive at any time, specifically on areas across the southern belt, from east to west."