The bloodshed began at dawn as a short-lived ground offensive in villages surrounding the provincial capital, also called Sweida, and was accompanied by waves of bombing in the city.
In the city, three Islamic State militants detonated suicide vests while explosions rang out from clashes in the eastern countryside of Sweida province, according to the Britain-based Syrian Observatory for Human Rights monitoring group.
Later in the day, a fourth suicide bomber sprayed the streets of the city with bullets before detonating his load.
At least 215 people were killed and 180 wounded, the head of the local health authority told pro-government media. The Observatory put the death toll at 221, including 127 civilians. Photographs suggested at least one Islamic State militant was later hanged from a bridge in Sweida city.
The Islamic State asserted responsibility for the violence, describing the militants as “soldiers of the Caliphate.” Although the group has largely been defeated across Syria and Iraq, it still holds pockets of territory.
In Sweida province, these sit on the eastern and western fringes, and the Islamic State has been gaining ground in recent days as rebel forces lay down their weapons under a surrender deal with the government.
Clashes between a local defense force and the Islamic State continued throughout Wednesday in the province’s eastern countryside, with the militants eventually losing their foothold in the villages they had entered in the morning. Although local fighters said the Syrian army had not been involved, the official Syrian Arab News Agency claimed later that government soldiers had. But on social media, some residents pushed back, questioning why the army’s presence hadn’t been larger at such a sensitive time for the region.
In the past, Islamic State militants have used similar attacks to foment sectarian tensions, a fault line the group was able to capitalize on during its 2014 sweep to power across swaths of Syria and Iraq. Although Sweida is a largely Druze city, its residents have taken in thousands of Sunni Muslims displaced by fighting in other parts of Sweida province.
The Islamic State’s slivers of territory are the last in southwestern Syria that remain outside of government control, and troops have been diverted from the province in recent months to bolster a military campaign against rebel forces, now defeated, across the region.
Zakaria Zakaria in Istanbul and Ghalia Al Alwani in Beirut contributed to this report.