The opposition group, the Syrian Observatory for Human Rights, based in London, said in a statement that clashes erupted at dawn in and around the city of Daraa, where the antigovernment uprising began in March. It said the attackers, armed with rifles and rocket-propelled grenades, hit two checkpoints in the countryside and a military base inside the city, suggesting a level of coordination that had not been seen there before.

The Syrian Observatory, which has a network of contacts and informants inside the country, did not specify the sources of the information, and it was unclear from the group’s statement whether any of the attackers were killed.