Fresh fighting has broken out between President Bashar al-Assad's forces and rebels rocking the capital Damascus and Syria's largest city, Aleppo.

Syrian forces pounded rebel positions in Aleppo, and explosions and gunfire have been reported across the region, including the outskirts.

Reports suggest a military jet roared across Aleppo followed by a huge explosion. Putting up a stiff fight, the rebels held on to the important district of Salaheddine in Aleppo. They also reportedly seized police stations to loot arms and ammunition.

Assad's forces, supported by helicopter gunships and tanks, stormed into Aleppo on 3 August in a bid to capture Salaheddine, a vital route for troops coming from the south.

Unverified online videos testify to the brutality of the Syrian regime.

"The dead are more noticeable now; bodies are lying on the streets, and a stench comes from those who have been buried under the debris. There is echoing gunfire all around. No one is quite sure who is shooting at whom and there is an intimate proximity of the two sides," wrote Kim Sengupta of the Independent newspaper who was an eye witness.

Dozens of rebel fighters and civilians are believed killed by security forces in the last few days, although the exact number is not known.

The Syrian Observatory for Human Rights said at least 110 people had been killed across Syria on 3 August, of whom 88 were civilians.

Meanwhile, reports suggest Iranian troops have joined the regime's forces in fighting against the rebels. One activist leader claimed that more than 3,000 Iranian snipers had been drafted by the Syrian regime.

Forces loyal to Assad are now reportedly conducting house-to-house searches for rebels in the region.

"The focus two weeks ago was on Damascus. The focus is now on Aleppo, where there has been a considerable build-up of military means, and where we have reason to believe that the main battle is about to start," the UN Under-Secretary-General for peacekeeping operations, Herve Ladsous, said.

WARNING: THE IMAGES BELOW CONTAIN GRAPHIC SCENES OF VIOLENCE, BLOODSHED AND DEATH.

Bodies of civilians, whom activists say were killed by forces loyal to Syria's President Bashar al-Assad, are seen in Aleppo's district of Salah EdinneReuters
A young Free Syrian Army fighter rests on a school bench on a street in downtown AleppoReuters
Damaged building are seen in Aleppo's district of Salah EdinneReuters
The body of a civilian, whom activists say was killed by forces loyal to Syria's President Bashar al-Assad, is seen in front of the wreckage of a bus in Aleppo's district of Salah EdinneReuters
Free Syrian Army fighters take a break from fighting with forces loyal to Syrian President Assad in downtown AleppoReuters
A Free Syrian Army fighter is wounded during a fight with forces loyal to Syrian President Bashar Al-Assad in downtown AleppoReuters
Residents gather during a mass burial of people who they say are opposition activists killed by forces loyal to Syria's President Bashar al-Assad, in Jdeidet Artouz near DamascusReuters
Bodies of Bashar al-Assad opponents, slaughtered outside DamascusReuters
People gather during the mass burial of people whom activists say were killed by forces loyal to Syria's President Bashar al-Assad, in ArtouzReuters
A boy reacts next to the body of Mamoun Al-Masalmeh whom activists say was killed by forces loyal to Syria's President Bashar Al-Assad before his funeral in DeraaReuters