BEIRUT A Syrian army tank rumbles past the entrance to a narrow city street, wreathed in oily black smoke apparently the result of a rocket-propelled grenade strike. A soldier leaps from the turret to escape the inferno.
Video footage posted by rebel fighters shows them exulting in this local victory. The images also illustrate the difficulty the army faces as it counter-attacks to drive insurgent fighters from central districts of the capital, Damascus.
While the army has an overwhelming advantage in fire-power, streets so narrow that tanks cannot bring their guns to bear on the enemy are not an ideal battleground on which to face an agile and opportunistic guerrilla force.
For their part, the rebels are able to embarrass the army in these street battles, but they are not well enough armed or commanded to capitalise on their gains. If the rebels can knock out tanks in Damascus, the army has thousands more.
The 16-month-old effort to overthrow President Bashar al-Assad reached what many commentators saw as a potential tipping point last week when a bomber infiltrated a meeting of security officials and killed four of Assad's most senior lieutenants.
Forty-eight hours later, the Syrian military began to drive the rebels out of central Damascus, district by district. The operation brought quick successes for the army but revealed strengths and weaknesses on both sides of the struggle.
While the rebels surprised the government with their advance into the capital, taking on troops within sight of the presidential palace, they are armed for the most part only with assault rifles, machineguns and grenade launchers.
Rebel sources said up to 2,500 fighters from outside Damascus had been infiltrated into the capital for a concerted push, but they have so far failed to capitalise on the bomb attack as their supporters had hoped.
As the assault by the army's 4th Division, commanded by Assad's feared brother Maher, got under way, the rebels fell back, demonstrating that they are unable to hold ground in the face of artillery and helicopter gunships.
At the same time, by withdrawing quickly, often through fields and farmland adjacent to central Damascus, the rebels were able to escape the might of one of the best-equipped armies in the region. The same story emerges in Aleppo, Syria's commercial hub, now the scene of bitter fighting.
"Yes, the rebels have retreated," said Brigadier General Fayez Amr, an army defector now on the side of the insurgents. "When you face such artillery and aerial bombardment, and long-range missile attacks, what choice do you have?
"But this will not crush the revolution and this is a war that Assad cannot win. Does the tactical retreat mean he has crushed six million people in Damascus?"
How close the rebels are to dislodging Assad remains unclear, although his position looks shakier than ever.
The president himself has not spoken in public since the bomb attack on his security cabinet.
He has taken a different tack from that adopted by Libya's Muammar Gaddafi, who made defiant speeches almost to the last, or Iraq's Saddam Hussein, who used his information minister, derided in the West as Comical Ali, to issue delusional statements claiming all was well as American troops invaded.
The 46-year-old Assad last month ordered his government to spare no effort to defeat rebels he has described as terrorists.
Recalling his earlier profession as an eye surgeon, he said: "When a surgeon performs an operation to treat a wound do we say to him: 'Your hands are covered in blood'?"
"Or do we thank him for saving the patient?"
Military experts say that with the insurgency on the march, Assad is being forced to concentrate his forces to protect important centres such as Damascus and Aleppo, even if this means losing control of outlying areas and some border posts.
Assad is deploying elite military units, whose loyalty can be relied on, to do most of the fighting. These are mostly Alawite, belonging to the same minority sect as the president, as are the feared shabbiha militiamen who are sent in to mop up resistance after artillery bombardments.
Desertions of dozens of senior military officers are seen as embarrassing but unlikely to make a critical operational difference just yet.
"Assad knows that he ... needs to be able to control main logistical and transport routes between Syria's urban hubs - Damascus, Aleppo, Latakia," said Anthony Skinner, Middle East expert at British political risk consultancy Maplecroft.
"To use the medical analogy, these are the arteries feeding the main organs - the heart and brain. Bashar al-Assad is clearly investing considerable fire-power to protect the centres of command, or main organs."
But with the insurgency growing in strength, Assad is forced to confront the rebels in an increasing number of locations at a time when his grasp is diminishing.
Local rebel commanders report indiscriminate shelling in some areas, but brush this off, saying it costs them few casualties while at the same time ensuring that the government incurs the wrath of people forced to flee their homes.
"It's a scorched earth policy to terrify people and to crush the revolution," said Brigadier Amr.
Mustafa Abdullah, who heads a unit called the Martyrs of Aleppo, told Reuters: "The regime is indiscriminately shelling areas around Aleppo -- Azaz, Tel Rifaat and Anadan -- but God willing we are steadfast."
How steadfast they can remain might be in question, as Abdullah complains that his men are running out of bullets, something that will inevitably hamper their aim of wearing down government forces with hit-and-run attacks.
The rebels also face a backlash from government forces desperate to halt the insurgency in its tracks.
"What we will be witnessing now is the beginning of a stage of even bloodier retaliation by the regime because the two cities they have long seen as their bastions, Damascus and Aleppo, are now at the heart of the battle," said Major Abu Abdul Rahman, a rebel brigade commander from Rastan.
"Assad's elite forces are resorting to escalating force to bring people to submission," he said.
While the rebels are convinced that their campaign will be settled on the streets of Damascus, some analysts speculate that Assad's forces might retreat to their Alawite heartland around the port of Latakia. A breakaway Alawite enclave is not however seen as viable in the long term.
While the fighting has been restricted to conventional weapons, Syria alarmed the West this week when it confirmed it had stocks of chemical weapons.
The government says it will not use these weapons against its own people, only against foreign enemies. Given the level of international disapproval, any use of these weapons would leave Assad even more isolated than he is now.
(Additional reporting by Suleiman al-Khalidi, Erika Solomon and Peter Apps; Editing by Alistair Lyon)