Syrian army tanks shelled Aleppo and a helicopter gunship strafed rebel positions with heavy machinegun fire as they fought for control of the country's biggest city and key battleground of the 17-month uprising.
After UN Security Council paralysis on Syria forced peace envoy Kofi Annan to resign last week, and with his ceasefire plan a distant memory, rebels have been battered by the government onslaught in Aleppo and the capital Damascus.
Reuters witnessed fierce street fighting in the Salaheddine district, a gateway into the city of 2.5 million people and the main focus of fighting in the past week.
Tanks pounded alleyways where rebels sought cover. One shell hit a building next to the Reuters reporting team, pouring rubble onto the street and sending billows of smoke and dust into the sky.
State television said Assad's forces were "cleansing the terrorist filth" from the country, which has been sucked into an increasingly sectarian conflict that has killed some 18,000 people and could spill into neighbouring states.
In Damascus, troops backed by fighter jets kept up an offensive on Saturday that they began a day earlier against the last rebel bastion there, a resident said.
Both cities - vital prizes in the battle for Syria - were relatively free of violence until last month, when fighters poured into Damascus shortly before a July 18 bomb killed four of Assad's inner circle, and entered Aleppo days later.
Rebel commanders say they anticipate a major Syrian army offensive in Aleppo and one fighter said they had already had to pull back from some streets after army snipers advanced on Saturday under cover of the fierce aerial and tank bombardment.
"The Syrian army is penetrating our lines," said Mohammad Salifi, a 35-year-old former government employee. "So we were forced to strategically retreat until the shelling ends," he said, adding the rebels were trying to push the army back again.
Once a busy shopping and restaurant district where residents would spend evenings with their families, Aleppo's Salaheddine district is now white with dust, broken concrete and rubble.
Tank shell holes gape wide on the top of buildings near the front line, and homes of families and couples have been turned into look outs and sniper locations for rebel fighters.
Large mounds of concrete are used as barriers to close off streets, the smell of gunfire and rotting garbage intermix. Lamp posts lie horizontally across the streets after being downed by shelling, their wiring swinging idly in the wind.
Civilians trickle back to collect their belongings and check on their homes. Late on Saturday a confused elderly man stumbled into 15th street as rebels exchanged fire with the army.
"Get out of the way! Get off the street!" fighters shouted, grabbing him and taking him to shelter from sniper fire.
"I just wanted to buy some blackberry juice," he told the fighters, his face reflecting confusion and horror at the damage to his street. Instinctively, he took his personal ID out of his chest pocket to show the rebels, a habit from the strict days of the Assad security officials.
During the day, others emerged from damaged buildings. A couple stood shaking with fear at an intersection a few meters from the fighting as a medic waved a car down to help take them to safety.
"Just to hold power he is willing to destroy our streets, our homes, kill our sons," wept Fawzia Um Ahmed, referring to Assad's determined counter-offensive against the rebels.
"I can't recognise these streets any more."
Outgunned by Assad's forces, the rebels are constantly on the hunt for captured weapons.
On one Salaheddine street, a rebel fighter drove up in a pick-up truck mounted with an anti-aircraft weapon, one of 15 that rebels said were seized during fighting last week.
But the weapon could not be aimed at the sky and neither did it fire. It remained parked on a side street.
"We only have 200 rounds per weapon," said Abu Furat al-Garabolsi, an army officer who defected. "We have to be totally sure we will shoot a plane when we fire it otherwise we won't be able to replenish what we have used."
After Annan's resignation, the U.N. General Assembly voted on Friday to condemn the Syrian government and criticise the U.N. Security Council's failure to agree tougher action, in a resolution that Western diplomats said highlighted the isolation of Assad supporters Russia and China.
Russia called the vote a "facade of humanitarian rhetoric" behind which Assad's foreign enemies were arming the rebels and worsening the violence that has elements of a proxy war between Sunni and Shi'ite Islam which could spill beyond Syrian borders.
Assad is a member of the Alawite faith, an offshoot of Shi'ite Islam that has dominated Syrian politics through more than 40 years of his family's rule in a country that has a Sunni Muslim majority.
The mostly Sunni Muslim Gulf Arab states have called for rebels to be armed and Turkey has provided them with a base, angering Damascus and prompting Syrian state television on Sunday to refer to the rebels as a "Turkish-Gulf militia".
It said the bodies of Turkish and Afghan fighters had been found in Aleppo, without giving details.
Assad still has the backing of Shi'ite Iran and Lebanon's armed Shi'ite Hezbollah movement. Tehran warned on Sunday against foreign intervention in Syria and said the conflict could engulf Israel.
"The fire that has been ignited in Syria will take the fearful (Israelis) with it," parliamentary speaker Ali Larijani said, according to the Islamic Republic News Agency.
On Sunday Syrian rebels claimed responsibility for kidnapping 48 Iranian pilgrims in Syria and said they were checking their identities to show that Tehran was involved in fighting for Assad, a rebel officer said.
The interview was aired after the Dubai-based al-Arabiya television broadcast a video showing armed men checking the identity cards of the kidnapped Iranians.
The fighters were "still checking the documents that prove the identity of these detainees and will make our findings public in due course," said a man identified as Captain Abdel Nasser al-Shumair, commander of the al-Baraa brigade of the Free Syrian Army. He said his men had been tracking the abducted Iranians for two months before they were seized.
Tehran has asked Turkey and Qatar to help secure the release of kidnapped men, Iran's state news agency IRNA said.
