Rebels fighting government forces in Syria's commercial capital Aleppo "will definitely be defeated," Syrian Foreign Minister Walid al-Muallem said on an unannounced visit to key ally Iran on Sunday. "We believe that all the anti-Syrian forces have gathered in Aleppo to fight the government... and they will definitely be defeated," he told a joint news conference in Tehran with Iranian Foreign Minister Ali Akbar Salehi. His assertion came as Syrian troops, backed by airpower and tanks, pushed on in the second day of an assault on the northern city of Aleppo, sparking international fears of a humanitarian catastrophe. Salehi warned that, if the Syria conflict worsened and the regime of President Bashar al-Assad fell, the consequences "would engulf the region and eventually the whole world." He added that "it's naive and illusory to think that, if a (power) vacuum opens up in Syria and the government changes, a new government could be easily established." He urged Syria's neighbours to think hard about the situation, otherwise "everyone will lose."