The Syrian Observatory for Human Rights, a Britain-based war monitoring group, said 156 people were killed.
Hayat Tahrir al-Sham, a group made up mostly of ex-members of the former al-Qaeda affiliate in Syria, also has a few hundred men in the south.
Clashes between Syrian government forces and Islamic State militants then continued throughout the day in the eastern countryside of Sweida province.
IS says in a statement posted on the group's social media channels that its "soldiers" carried out surprise attacks on government and security centers in Sweida on Wednesday.
The Russian Defense Ministry thanked Israel on Thursday for destroying Islamic State (IS) positions and killing terrorists after two rockets were fired into Israel from Syria's embattled south Wednesday evening.
At least 41 civilians have been killed in air strikes on the jihadist holdout since July 19, the monitor says.
It put the death toll for those attacks and others elsewhere in the surrounding province, also called Sweida, at 38.
The government controls nearly all of Sweida province but IS retains a presence in desert areas of its north and east.
The decision to hit the IS launcher came a week after Israel signaled it was would not interfere with Assad's retaking of the Syrian Golan Heights and would seek to make sure the 1974 agreement demarcating the separation of Israeli and Syrian forces be kept.
They said the attacks that killed over two hundred people, the worst against the majority Druze inhabited city in southwestern Syria that escaped violence in over seven-ear of conflict, sent shockwaves across the Druze community spread in Lebanon, Syria and Israel.
After decades of calm, tensions began flaring up in the Golan with the 2011 uprising against Syrian President Bashar al-Assad's regime.
After losing its strongholds in eastern Syria previous year, IS launched insurgency operations from pockets of territory in desert areas.
U.S. prosecutors in IN said Musaibli was transferred from the custody of the Syrian Democratic Forces along with a United States woman, Samantha Elhassani, who has been charged with making false statements to the Federal Bureau of Investigation.
President Bashar al-Assad's forces are now fighting to capture the last-remaining rebel villages in the southern part of the Golan Heights.
And the Russian foreign ministry said the IS attacks "confirm the need for energetic and coordinated efforts by the global community to eradicate this universal evil from Syrian territory".
'After that, the regime will have complete control of the province of Quneitra and the line dividing it from the Israeli-occupied Golan'.