Syrian President Bashar al-Assad held unannounced talks with a top envoy from closest ally Iran on Tuesday, as his troops engaged rebels in fierce fighting in key battleground city Aleppo, and Syrian rebels said they were running low on ammunition because Assad's troops encircled their stronghold at the southern entrance to the country's biggest city.

Iran, which has voiced growing criticism of support by the United States, Turkey, Saudi Arabia and Qatar for the rebels fighting Assad's forces, also sent its foreign minister to Ankara and a stern letter to Washington holding them responsible for the fate of 48 of its citizens kidnapped by the rebels.

But the rebels say they suspect the captives were troops sent to help Assad. A rebel spokesman in the Damascus area said on Monday that three of the Iranians had been killed by government shelling, and the rest would be executed if the shelling did not stop.

Saeed Jalili, a top aide to Iran's supreme leader Ayatollah Ali Khamenei, went straight into a meeting with Assad on his arrival from neighboring Lebanon.

"The Islamic Republic of Iran believes in national dialogue between all domestic groups to be the solution, and believes foreign solutions are not helpful," Iran's state media quoted him as saying on arrival in Damascus.

"We hope to take an effective step in regards to this new direction."

Assad has reinforced his troops in preparation for an assault to recapture rebel-held districts of Aleppo after repelling fighters from most of Damascus.

"The Syrian army is trying to encircle us from two sides of Salaheddine," said Sheikh Tawfiq, one of the rebel commanders, referring to the southwestern neighborhood that has seen heavy fighting over the last week.

Mortar fire and tank shells exploded across the district early on Tuesday, forcing rebel fighters to take cover in crumbling buildings and rubble-strewn alleyways.

Tanks have entered parts of Salaheddine and army snipers, using the cover of heavy bombardment, deployed on rooftops, hindering rebel movements.

Another rebel commander, Abu Ali, said snipers at the main Saleheddine roundabout were preventing the rebels from bringing in reinforcements and supplies.

But rebels said they were still holding the main streets of Salaheddine that have been the front line of their clashes with Assad's forces.

PM defects

As Assad's forces battle to retake Aleppo, the president has suffered a series of setbacks including on Monday when Prime Minister Riyad Hijab denounced the "terrorist regime" in Damascus after fleeing the country.

Opposition figures, despite setbacks in recent weeks of fighting in the two main cities Damascus and Aleppo, spoke of an extensive and long-planned operation to spirit Hijab and his large extended family across the border to Jordan.

"I announce today my defection from the killing and terrorist regime and I announce that I have joined the ranks of the freedom and dignity revolution," Hijab said in a statement read by a spokesman. He declared himself "a soldier in this blessed revolution".

But Syrian state television said on Monday that Hijab had been fired.

A spokesman for US President Barack Obama hailed Hijab's defection as a sign that the 40-year rule of Assad's family was "crumbling from within" and said he should step down.

Western leaders' repeated predictions of Assad's imminent collapse have so far proven premature, however.

Reuters-AFP