Twenty-one people, including four children and six women, were killed when a minibus hit a mine and exploded in the southern Syrian town of Noa on Wednesday, the Syrian Observatory for Human Rights monitoring group said.
Opposition activists told the Observatory the minibus drove over a mine planted by forces loyal to President Bashar al-Assad. There was no immediate comment from the government.
The explosion was reported in rebel-held territory in Deraa province but there are also army troops in the nearby base of Tel al-Jumaa, which is besieged.
Assad's forces are battling an uprising that grew out of protests against his family's four-decade grip on the country.
Violence has continued in recent days despite pleas from Arab and Muslim organisations for a ceasefire to mark the Muslim festival of Eid al Adha.
The British-based Observatory, which is opposed to Assad, reported clashes in most provinces on Wednesday and said war planes had been deployed to the eastern desert city of Deir al-Zor.
The group said at least 27 government soldiers had been killed during intense clashes in Deir al-Zor over the past two days although rebels gave a figure more than double that.
Pro- and anti-government militias have fragmented the country into fiefdoms, with hardline Islamist rebels fighting Kurds and other opposition groups.
The Observatory reported that 29 jihadist fighters, some from the al Qaeda-linked Islamic State of Iraq and the Levant, and 12 Kurdish militants were killed during clashes on Tuesday in the northeast province of Hasaka.