Thousands of Iraqi and Lebanese fighters have arrived in the Syrian city of Aleppo to support regime forces in their fight against the opposition, Iran's semi-official Fars News Agency reported.
Lebanon's Hezbollah deployed fighters from its Radwan division to a government-held neighbourhood in Aleppo.
Meanwhile, reinforcements are also arriving from Iraq via Iran from the Iraq-based Hezbollah Brigades, Asaib Ahl Al-Haq and Harakat Al-Nujaba, the Associated Press reported militia officials as saying.
The Hezbollah Brigades have deployed 1,000 fighters on Sunday, an official said. Harakat Al-Nujaba announced on Facebook that it sent 2,000 fighters.
Iran and its allies have lost strategic sites in the battles, Fars reported, adding that the US fleet in the Mediterranean provides the opposition with information on the movement of Syrian troops and Iranian militias.