"Today's crime shows that countries supporting terrorism are trying to breathe life back into the terrorist organisation to keep it as a card in their hand that they will use to achieve political gains", Syrian President Bashar al-Assad as he received Russia's envoy to Syria Alexander Lavrentiev.
Al-Ikhbariya said one of the attackers hit at a vegetable market in the city of Sweida just after 5 a.m., a busy time for the merchants at the start of their day.
The Britain-based Syrian Observatory for Human Rights says that 302 people, including 135 civilians, were killed.
Damages after a suicide bomb attack are seen in Sweida, Syria July 25, 2018.
In the eastern countryside of Sweida, some areas in the remote eastern countryside are under IS control.
Pro-Assad forces raised the Syrian flag in the largely ruined city of Quneitra on Thursday, regained from surrendering rebels as the government tightens its hold on the Syrian sector of the Golan Heights, strategic territory bordering Israel and Jordan. Most of Daraa province is controlled by opposition forces, though the government holds parts of the provincial capital and a few villages in the northwest. At least 45 jihadists died carrying it out, the Observatory said.
Daesh lost almost all the territory it once held in Syria previous year in separate offensives by the Russian-backed army and a USA -backed militia alliance.
Tour de France 2018 - Julian Alaphilippe wins Stage 16 amid Tour protests
Police were said to be arresting some protesters after farmers threw hay bales across the road before the peloton's arrival. Stage 17 is a 65 km trek from Bagneres-de-Luchon to Saint-Lary-Soulan on Wednesday.
It said dozens of IS militants were also killed in the clashes.
Hezbollah offered condolences to the people of Sweida in particular and all Syrians in general, as he praised the courage of the Syrian Army soldiers and the locals who confronted the terrorist attacks and foiled them.
With the help of Russian air power, the Syrian army has been hitting IS in a separate pocket further west, near the frontier with the Israeli-occupied Golan Heights.
The assaults, it said, come "in the wake of the recent victories by Syria and its allies, especially in the south of the country".
The Syrian military, backed by Russian forces, recently launched an operation to drive rebel from their remaining strongholds in the south-west.
In the northeastern countryside of Sweida, the IS militants launched simultaneous attacks on several towns amid confrontation between the Syrian army and the IS militants in an attempt by the army to restore the points taken by IS, according to the observatory.