Car bombs and rocket fire shook Aleppo Saturday as rebels battled to break a suffocating siege by the Syrian regime, accused by Washington of using starvation as a "weapon of war".
The offensive, launched Friday, aims to break through a three-month encirclement of the battered city's eastern districts, where more than 250,000 people live without access to food or humanitarian aid.
"In just a few days, we will open the way for our besieged brothers," rebel commander Abu Mustafa told AFP from the frontline district of Dahiyet al-Assad, on the southwestern outskirts of Aleppo.
Fighting and air strikes pounded nearly all of Aleppo's western outskirts, with the most intense clashes reported in the districts of Al-Zahraa and Dahiyet al-Assad.
Yasser al-Youssef of the Noureddin al-Zinki rebel faction said opposition fighters opened a new front in Al-Zahraa on Saturday with a massive car bombing.
The Syrian Observatory for Human Rights monitoring group said rebels and allied Islamist militants have unleashed a barrage of rocket fire and at least 10 car bombs since their assault began.
The Britain-based group said two days of fighting have killed at least 30 regime forces and allied fighters as well as 26 Syrian rebels, but it did not give a toll for foreign militants battling alongside the opposition.
At least 21 civilians, including two children, have been killed in rebel bombardment since Friday morning.
Syrian state news agency SANA said rockets fired by opposition groups on Saturday wounded six people including a child in two regime-held districts.
The offensive has seen more than 1,500 rebels from the provinces of Aleppo and nearby Idlib amass along a front stretching for 15 kilometres (nine miles) down the city's western edges.
Their aim is to work their way east through a sprawling military complex, then to the district of Al-Hamdaniyeh to break through government lines.
Fighting on Saturday was so fierce around Al-Zahraa and Dahiyet al-Assad that the explosions and gunfire could be heard across Aleppo's eastern half, AFP's correspondent there said.
"There have not been clashes this intense in Al-Zahraa since 2012," when opposition fighters seized Aleppo's east, said Observatory head Rami Abdel Rahman.
He said pro-government forces launched a counter-attack on Saturday and managed to recapture several positions in Dahiyet al-Assad, where rebels had scored a major advance.
An AFP correspondent who visited the district saw deserted streets and extensive damage to buildings battered by air strikes and artillery fire.
Syria's second city, Aleppo has been devastated by some of the heaviest fighting of the country's five-year civil war that began with anti-government protests and has since killed more than 300,000 people.
Much of the once-bustling economic hub has been reduced to rubble by air and artillery bombardment, including barrel bombs -- crude unguided explosive devices that cause indiscriminate damage.
Last week, Russia implemented a three-day "humanitarian truce" intended to allow civilians and surrendering rebels to leave the east, but few did so.
Russia, whose intervention in September 2015 with air strikes in support of President Bashar al-Assad's forces was seen as a game-changer, says it has not bombed Aleppo since October 18.
The Observatory said Saturday that Russian raids have been battering Aleppo's western battlefronts, but confirmed the halt to Moscow's aerial bombing of the city itself was holding.
The Russian military said Friday it had asked President Vladimir Putin for authorisation to resume the raids.
But Kremlin spokesman Dmitry Peskov said Putin "considers it inappropriate at the current moment", adding that the president thought it necessary to "continue the humanitarian pause" in Aleppo.
The United States on Friday accused the regime of using starvation as a weapon of war -- a war crime under the Geneva Conventions.
Rejecting Kremlin claims that attacks on Aleppo have stopped, a US official told AFP that "the regime has rejected UN requests to deliver aid to eastern Aleppo -- using starvation as a weapon of war".
Aleppo's front line runs through the heart of the city, dividing rebels in the east from government troops in the west.
It lies at the crossroads of key transport routes, making it a strategic prize for both sides and a potential bargaining chip for both opposition and regime forces if UN-brokered peace talks are resumed.