The death toll rose throughout Wednesday as Daesh carried out a targeted assault in the south of the country. Half of those killed were civilians, the Syrian Observatory for Human Rights said.
The death toll in coordinated Daesh suicide bombings and shootings in southern Syria rose to nearly 250 overnight, more than half of them civilians, a war monitor said Thursday.
Wednesday's attacks hit Sweida, a Druze-majority province under regime control for the most part and until now relatively insulated from the war, now in its eighth year.
The death toll climbed steadily throughout the day and into the night, the Syrian Observatory for Human Rights (SOHR), a Britain-based monitoring group, said.
"The toll is now 246 people dead, including 135 civilians," said SOHR chief Rami Abdel Rahman.
The others killed were pro-regime fighters or residents who had taken up arms to defend their villages.
"The toll keeps rising as civilians who were wounded are dying and people who were unaccounted for are found dead," Abdel Rahman said.
The onslaught began with a triple suicide bombing in the city of Sweida, which was followed by attacks with guns and explosives on villages to its north and east.
A fourth blast hit the provincial capital later in the day.
Daesh claimed the assault hours later. At least 45 militants died carrying it out, the war monitor said.
It was the worst bloodshed to hit Sweida province since the civil war began in 2011.
Israel hits Syria after errant rocket fire
The site of Wednesday's attacks by Daesh is near a region where Damascus is fighting to take areas of south and southwestern Syria from rebels.
The Israeli military on Wednesday said it hit a rocket launcher across the border, in response to two missiles that fell into the Sea of Galilee.
The army said in a statement late on Wednesday that the area surrounding the rocket launcher was targeted by Israeli artillery.
The Associated Press reported that the rockets appeared to be errant fire from clashes between pro-regime forces in Syria battling rebels near Syria's frontier with Israel.
Earlier this week Israel shot down a Syrian fighter jet that it said had breached Israeli airspace.