At least 69 people were killed in southern Syria on Monday (local time), most of them in clashes between army deserters and troops loyal to President Bashar al-Assad, activists said on Tuesday.
The violence came as Syria faces growing international isolation following the Arab League's decision to suspend its membership in response to Assad's crackdown on eight months of protests calling for his overthrow.
Hundreds of people have been killed so far this month, making it one of the bloodiest periods of the Syrian protests, inspired by uprisings which have overthrown leaders in Tunisia, Egypt and Libya.
The Syrian Organization for Human Rights said 34 soldiers and members of Syrian security forces were killed in clashes with suspected army deserters who attacked military vehicles in the southern province on Monday.
Video footage broadcast by Al Jazeera television showed what appeared to be a tank engulfed in flames, alongside other burning vehicles.
At least 12 of the attackers were also killed, the British-based Observatory said, while another 23 people were killed in "gunfire from security and military checkpoints" in southern villages.
Syria has barred most foreign media from the country, making it difficult to verify conflicting accounts from authorities and activists. Syria's state news agency SANA made no mention of clashes in the south on Monday.
The United Nations says 3,500 people have been killed in Assad's crackdown on the protests and human rights groups say security forces have carried out killings and torture which constitute crimes against humanity.
Authorities blame armed groups for the violence, saying at least 1,100 soldiers and police have been killed since the uprising broke out in March.
Alongside street protests, which rights groups say have been mainly peaceful, an increasingly forceful armed insurgency has targeted Assad's military and security forces.
Monday's violence took place in the southern province of Deraa which borders neighbouring Jordan and was the region where major protests first erupted against Assad in March.