"There was a big blast at dawn, most likely due to a car bomb attack by the rebels against a regime position between the districts of Jobar and Qabun," Observatory chief Rami Abdel Rahman told AFP, adding that fierce clashes continued during the morning.
Syria's air force responded with raids on opposition positions while rebels shelled the Abbasids and Tijara neighbourhoods near Jobar, a few kilometres (miles) from the city centre.
The official SANA news agency said that 12 people were wounded in rebel shelling.
An AFP journalist in eastern Damascus reported hearing a large explosion at dawn before clashes broke out.
On Sunday, regime forces, using warplanes and ground forces, succeeded in repelling a rebel push on eastern neighbourhoods of the city.
The Islamist Faylaq al-Rahman rebel group and the Fateh al-Sham Front -- known as Al-Nusra Front before it renounced its ties to Al-Qaeda -- have a presence in Jobar.
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