Syrian government troops and rebels fought pitched battles in several areas of the country on Saturday, the second day of a ceasefire marking the four-day Muslim festival of Eid al-Adha, activists reported.
At least 20 people were killed in the violence, heightening concerns that the truce brokered by international peace envoy Lakhdar Brahimi might collapse, DPA reported.
The latest victims include five civilians who died in a car bombing in the eastern province of Deir al-Zour, said the opposition Syrian Observatory for Human Rights.
Eight others were killed in a shelling attack in the outskirts of the capital Damascus, added the Britain-based organization.
Fighting between troops loyal to President Bashar al-Assad and rebels was reported to have taken place in the town of Deir al-Zour and in the northern provinces of Aleppo and Idlib.
Tension has also gripped the predominantly Kurdish-Christian area of Ashrafiyeh in Aleppo, a day after Kurdish fighters barred rebels battling al-Assad's troops from the area, said the Observatory.
It added that 19 rebels and 10 local Kurdish gunmen were killed in a clash in the area on Friday.
Kurds says the rebels had breached an earlier agreement, in which they had promised not to enter their areas.
Kurds have accused the militant Islamist group al-Nusra Front, which never endorsed the four-day ceasefire, of attempting to push into Kurdish and Christian areas and widening the battleground in Aleppo.
The Observatory warned in a statement that the recurrence of such clashes could trigger sectarian strife.
Neither Kurds nor Christians in Aleppo have taken sides since fighting for control of the city started three months ago.
Kurds are estimated to be Syria's biggest ethnic minority, making up around 10-15 per cent of the country's population of 23 million.
The Syrian Observatory for Human Rights said it had registered 220 breaches since the ceasefire went into effect on Friday, resulting in at least 150 deaths on that day.
"Neither side seems ready to stop. The truce looks set to collapse," Rami Abdel Rahman, the head of the organization, told dpa.
The Syrian army and rebel fighters have blamed each other for violating the ceasefire, which Brahimi had hoped would clear the way for a political solution to the country's 20-month conflict.