Twenty-one people, including four children and six women, were killed when a minibus exploded in the Syrian southern town of Noa early on Wednesday, the Syrian Observatory for Human Rights monitoring group said, according to Reuters.

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 Syria's Deraa province but there are also army troops in the nearby base of Tel al-Jumaa, which is besieged.

The British-based Observatory, which is opposed to Assad, reported clashes in most provinces on Wednesday and said that 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.

A car bomb killed at least 20 people on Monday in the northern Syrian town of Darkoush. The Observatory said dozens of people were wounded by the explosion in the market of Darkoush, a small rebel-controlled town 2 km from the frontier, and some of them were taken into Turkey for treatment.