Islamic State militants killed scores of people in a series of attacks on government-held parts of southwestern Syria today, including multiple suicide blasts in Sweida city, official sources said.

The seemingly coordinated attacks were the deadliest to hit government territory in many months. At least 50 people were killed and 78 wounded in total, the head of the Sweida health authority told al-Manar TV, run by Damascus ally Hezbollah.

The Syrian Observatory for Human Rights, a Britain-based war monitoring group, said at least 100 people were killed. Islamic State said in a statement that it had carried out the attacks.

Northeast of Sweida city, the jihadists launched simultaneous attacks on several villages where they clashed with government forces, state media and the Observatory said.

In the city itself, at least two attackers blew themselves up, one near a marketplace and the second in another district, state television said. State news agency SANA said two other IS militants were killed before they could detonate their bombs.

The Observatory said jihadists seized hostages from the villages they had attacked. It said the dead included at least 35 civilians.

Sweida Governor Amer al-Eshi said authorities also arrested another attacker. "The city of Sweida is secure and calm now," he told state-run Ikhbariyah TV.

Islamic State was driven from nearly all the territory it once held in Syria last year in separate offensives by the Russian-backed army and a US-backed militia alliance.

Since then, President Bashar al-Assad has gone on to crush the last remaining rebel enclaves near the cities of Damascus and Homs and swept rebels from the southwest.

The Observatory said government forces battled jihadists who stormed the villages from an Islamic State pocket northeast of the city.

Government troops and allied forces hold all of Sweida province except for that enclave.

The air force pounded militant hideouts northeast of the city after soldiers thwarted an attempt by Islamic State fighters to infiltrate Douma, Tima and al-Matouna villages, state media said.

The army and villagers regained control of a hill and broke a brief siege of another nearby village after clashes, Ikhbariyah said.

With the help of Russian air power, the Syrian army has been hitting Islamic State in a separate pocket further west, near the frontier with the Israeli-occupied Golan Heights.

The Yarmouk Basin in southwest Syria remains in jihadist hands, after an army offensive defeated rebel factions in other parts of the southwest. The operation has focused on Deraa and Quneitra provinces.