Fighting between government forces and rebels has been continuing in Syria's two largest cities – the capital, Damascus, and Aleppo.
Rebels claimed their forces now control half of Aleppo after six days of fighting.
No independent confirmation of the claim was available.
Reports spoke of government planes carrying out air strikes on the outskirts of Aleppo to prevent rebel reinforcements from arriving to join the battle.
Reports also said there were indications the government was sending reinforcements to Aleppo in an attempt to drive the rebels from the city.
In Damascus, activists in the south of the capital said heavy shelling by government forces had continued following last week's rebel bombing that killed four senior regime security officials and wounded others.
Meanwhile, the United States has confirmed the defection of two more Syrian diplomats.
Abdel Latif al-Dabbagh, ambassador to the United Arab Emirates, and his wife, Lamia Hariri, Syria's charge d'affaires in Cyprus, are reported to have fled to Qatar.
White House spokesman Jay Carney said on July 25 that the move showed senior officials were fleeing the government because of the “heinous actions” taken by President Bashar al-Assad and because of the recognition that his days were "numbered."
A military attache at the Syrian Embassy in Oman, Mohammed Tahseen al-Faqir, told Al-Jazeera television that he also had defected.
Syria's ambassador to Iraq, Nawaf Fares, left for Qatar earlier this month.