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.