Syrian army deserters killed at least 27 soldiers and security force personnel in a series of clashes in the southern province of Deraa at dawn on Thursday, the Syrian Observatory for Human Rights said.
The British-based group said the deserters fought forces loyal to President Bashar al-Assad in two locations in the city of Deraa itself, as well as a checkpoint at a crossroads about 25 km (15 miles) to the east of the city.
It did not say how the clashes broke out, but the high casualties among security forces suggested coordinated strikes by the army rebels, who have escalated their attacks on military targets in recent weeks.
Rami Abdulrahman of the Observatory said in the fighting near Musayfrah, east of Deraa, all 15 personnel at a joint army and security checkpoint were killed.
The United Nations says 5,000 people have been killed in Assad's crackdown on protests against his rule which erupted in Deraa nine months ago, inspired by uprisings elsewhere in the Arab world.
The protests have been increasingly overshadowed by the armed insurgency against Assad's forces. Authorities say armed groups have targeted civilians and security forces since the start of the uprising, killing more than 1,100 soldiers and police.
(reporting by Dominic Evans; Editing by Rosalind Russell)