Islamic State militants have killed more than 180 people in a series of gun attacks and suicide blasts on government-held parts of southwestern Syria.

Raids and suicide bombings carried out today in Syria's southern Sweida province have killed at least 183 people, including 89 civilians, a war monitor group said.

The Syrian Observatory for Human Rights, a Britain-based war monitoring watchdog, said they were killed following seemingly coordinated attacks, including multiple suicide blasts in Sweida city itself.

The remaining 94 dead were pro-regime fighters, mostly residents who picked up arms to defend their homes, it said.

Most of the deaths occurred in raids on villages in the northeast of the province, while the remainder were killed in suicide attacks in the provincial capital, the monitor said, updating earlier tolls which had initially indicated a much lower toll.

Fruit is seen scattered on the ground after a suicide bombing in Sweida, in southern Syria, on July 25

'It's the bloodiest death toll in Sweida province since the start of the war' and one of the deadliest ever IS attacks in Syria, Observatory head Rami Abdel Rahman said.

He added that regime forces eventually ousted IS from several villages its fighters had seized and put an end to the attacks.

'Some residents who fled the attacks on their villages are returning and finding people dead in their homes,' Abdel Rahman said.

At least 38 IS fighters were also killed, including the suicide attackers.

Islamic State said in a statement that it had carried out the suicide blasts and gun attacks, which are the deadliest to hit government territory in many months.

The coordinated attacks - the worst in recent months - had all the hallmarks of the militant group and were reminiscent of its horrific assaults that spread mayhem over the past years in Syria, already ravaged by civil war.

The suicide bombings in the city of Sweida, a provincial capital populated by Syria's minority Druze, were apparently timed to coincide with attacks by a militant group linked to IS on a number of villages in the province, also called Sweida.

Al-Ikhbariya state-run TV showed images from several locations in the province and its capital where the bombers blew themselves up.

Damage can be seen above following today's attacks, the deadliest to hit government territory in many months

It said one of the attackers hit at a vegetable market in the city of Sweida just after 5am, a busy time for the merchants at the start of their day.

The bomber drove through the market on a motorcycle and there detonated his explosives, the TV station said.

A second attacker hit in another busy square in the city. Two other attackers blew themselves up as they were chased by security forces.

The city of Sweida has largely been spared most of the violence that Syrian cities have witnessed in the years since the conflict started in 2011.

Northeast of Sweida, jihadists launched simultaneous assaults on several villages where they clashed with government forces.

State news agency SANA said that two other IS militants were killed before they could detonate their bombs.

Damage is seen after a suicide bombing in Sweida, in southern Syria

Sweida Governor Amer al-Eshi said authorities had arrested an attacker: 'The city of Sweida is secure and calm now,' he told state-run Ikhbariyah TV.

The extremist group claimed responsibility for today's deadly attacks, boasting that its 'soldiers' killed more than 100 people in Sweida.

In a statement posted on the group's social media channels, it said its militants carried out surprise attacks on government and security centers in Sweida, sparking clashes with Syrian troops and allied militias before detonating their explosive belts.

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 U.S.-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.