Dozens of government tanks have converged on Syria's largest city as president Bashar Assad marshalled his forces to stamp out a five-day rebel fight to wrest Aleppo from the regime's grasp.
As the fighting raged, Turkey said that it had sealed its border to trade with Syria but would keep the frontier open to civilians fleeing the violence or in search of supplies. Two more Syrian diplomats, meanwhile, defected in the latest sign of cracks in the upper echelons of the Assad regime.
Fierce streets battles have raged in Aleppo since Saturday as rebels have slowly pushed through friendly neighbourhoods on the outskirts of the city towards its ancient centre. Although the regime has brought in its superior firepower, including attack helicopters and fighter jets, its forces have yet to drive out the rebels without additional reinforcements.
A similar rebel assault in Damascus last week took days for the government to control, and only then with the help of artillery bombardments and helicopters.
Northern Syria, especially the province of Idlib near Aleppo, has seen some of the heaviest and steadiest fighting between government forces and the rebels, and large swathes of the countryside are under opposition control.
While Syrian government forces are stretched thin by fighting taking place across the country in cities like Homs and Hama in central Syria, Deir el-Zour in the west, Daraa in the south and Idlib province in the north, they can defeat any single rebel assault by concentrating their forces, as they now appear to be doing with Aleppo since pacifying Damascus.
Yet even as Syria's powerful 300,000-man-strong military holds fast in the battle against the rebels, there are signs of cracks among the elites of the regime with a string of recent high profile defections.
In another blow to the regime, Turkey sealed its border with Syria to trade, though it will remain open for Syrians fleeing or seeking supplies.
Turkey's economy minister Zafer Caglayan said that the deteriorating security was behind the closure of a border through which Turkey once exported food and construction materials to the entire Middle East.
In Damascus, the new commander for the 300-member UN observer force, Lt. Gen. Babacar Gaye, and the UN official for peacekeeping operations, Herve Ladsous, were assessing the prospects for a UN peace plan that has been widely ignored by both sides.