More than 20 other soldiers were injured in the pre-dawn clashes in the province of Dara, the Syrian Observatory for Human Rights told Iran's Press TV.
The fighting was reported in the town of Basr al-Harir, while heavy machine-gun fire was also heard in the town of Dael, but there were no casualty reports from there.
The fighting erupted hours before a team of Arab League observers who have been touring Syria were to present their findings in Cairo, the report said.
The recent presence of Arab League observers appears to have emboldened anti-government forces. Friday, a police bus in Damascus was bombed, killing at least 20 people and injuring 60 others.
Since March, armed protesters have taken on the police and military of President Bashar Assad, demanding he step down to allow for democratic reform. He has steadfastly refused and ignored international calls for him to stop using force against unarmed demonstrators.
The United Nations estimated in December at least 5,000 people have been killed in the uprising.
The Syrian government, which barred foreign media and never issued specific casualty numbers, responded to the U.N. report by saying 2,000 of the fatalities were government forces.