Government air and artillery bombardments hit rebel-held areas of the Syrian city of Deraa, on the border with Jordan, on Tuesday after a two-day ceasefire expired, witnesses, rebels and the army said.
Syrian state television quoting army sources said they had resumed the offensive, which took place as U.S. and Russian officials were holding another round of secret talks on creating a "de-escalation zone" in southwestern Syria that would include Deraa.
U.S. and Russian officials agreed a ceasefire, which ended on Monday, during talks in Amman aimed at strengthening goodwill before more detailed negotiations on setting up the "de-escalation zone", diplomats in Jordan said.
On Saturday the Syrian army said it would suspend combat operations in Deraa for 48 hours to support "reconciliation efforts".
The halt in hostilities had been widely expected to be extended with signs of progress in the talks in Amman and an agreement to continue the detailed discussions, diplomats said.
A rebel official said Russian jets played a major role in the intensive raids after the army and Iranian backed militias had resumed air and artillery bombardments in the city.
"They tried to disperse the rebel forces on several fronts.
This is a regime that does not understand truces and they will keep trying to attack but we are ready for them," said commander Abu Rida Abu Nabout, head of the Liwa al Tawhid rebel group fighting in the city.
Rebels in the city and other residents have said this month that the army's bombardment of Deraa has intensified, and Western backed rebels said the army and Iranian backed militias had brought more troops to the city in a possible prelude to a campaign to wrest full control of the city.
The army has failed to dislodge the rebels from their stronghold in Deraa city so far, however, and rebel forces have thwarted the army's drive to regain a major border crossing with Jordan and cut rebel supplies east and west of the city.
One witness said barrel bombs, artillery shells and rockets were used in the bombardment of Deraa.
Major Issam al Rayes, spokesman for the Southern Front grouping of the Free Syrian Army (FSA) rebels, said rebels had thwarted a government advance towards a rebel held military base southwest of the city near the Jordan border.
"They advanced toward the base but where then encircled by rebels and incurred heavy losses and had to retreat," Rayes said, adding that at least 20 army and militia fighters were killed in what he described as a failed offensive.
Syrian President Bashar al-Assad is backed in the six-year-old war by Russia, Iran and Shi'ite militias while some of the rebels seeking to oust him are supported by the United States, Turkey and Gulf monarchies.