A car bomb in a Damascus suburb has killed 10 people.
Syrian officials said the blast in Jaramana also wounded 41 people and caused heavy damage.
The UN tried to broker a cease-fire over the four-day Eid al-Adha Muslim holiday, but the truce collapsed almost immediately after it was supposed to take effect.
Meanwhile Syrian warplanes heavily bombarded rebel targets in the suburbs of Damascus in what activists said was one of the most intense air raid campaigns around the capital since the uprising began 19 months ago.
The army had warned that it would strike "remnants of terrorists with an iron fist" after they "repeatedly violated the cease-fire." The regime of president Bashar Assad often refers to those waging the uprising as "terrorists."
Mohammed Saeed, an activist based in the Damascus suburb of Douma, said there were at least 15 air raids on the suburbs early Monday. The Britain-based activist group Syrian Observatory for Human Rights said planes carried out at least six air raids on Damascus suburbs including Rankous and Harasta. It said there are intense clashes in those areas as troops try to regain control from the rebels.
The Observatory also reported clashes and shelling in other parts of the country including the north-western province of Idlib that borders Turkey, where it said planes carried out 11 air raids on several villages. Amateur videos showed warplanes in the skies, then giant mushroom clouds of smoke after the missiles hit.
UN Secretary-General Ban Ki-moon expressed deep disappointment at the collapse of the cease-fire and urged more unity from the international community.