Heavy clashes broke out on Monday between Syrian troops and rebels in Damascus and in the second city Aleppo, where a Red Crescent warehouse burned amid fighting, a watchdog and residents said.
Fighting erupted at dawn in the capital's southern districts of Al-Hajar Al-Aswad and Tadamun, near the Yarmuk Palestinian camp, the Syrian Observatory for Human Rights said.
"There were multiple coordinated attacks on military posts in the area before dawn. Clashes ensued and factions of Palestinian fighters joined in on both sides," said Observatory director Rami Abdel Rahman.
Regime jets bombarded the eastern outskirts of Damascus, areas in the northwest province of Idlib and Albu Kamal near the Iraqi border in the east, the Britain-based watchdog said.
The regime has increasingly relied on air raids to try to fight back rebel gains on the ground.
In Aleppo, fighting broke out in several neighbourhoods, including northwest Zahraa, said the Observatory, which relies on a network of activists, lawyers and medics on the ground.
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A source within the Syrian Arab Red Crescent said that the main warehouse of the relief organisation in Aleppo, located next to the air force intelligence building in Zahraa, was burned down.
"We haven't been able to reach our warehouse for over a week with the heavy fighting, but according to the guards the warehouse has burned completely," the source told AFP on condition of anonymity.
He said the warehouse contained medicine, food, baby formula, and critical winter relief items like blankets. The other Red Crescent warehouse in Aleppo is under rebel control in the eastern Sakhur district.
A pharmacist living just south of Zahraa said the clashes were the worst he had heard since fighting broke out in the area.
"It's been almost one week that we are living in terror at night. We hear everything -- gun battles, tank shelling, explosions. We were so afraid that we moved my sons from their room to the inside of the house," Samir, 37, told AFP.
Clashes also broke out amid shelling attacks in Syria's third city of Homs, where an army operation began several days ago, the Observatory said.
It said that 203 people, including 112 civilians, were killed in nationwide violence on Sunday, while 36,000 people have been killed in since the uprising against the rule of President Bashar al-Assad broke out in March 2011.