About 35 people have been killed in fighting between Syrian pro-government forces and suspected Islamic State militants at a strategically important military airport in Syria's south, opposition activists say.

The Syrian Observatory for Human Rights (SOHR) said the pro-government troops lost about 20 fighters, and at least 15 militants were killed.

It said the forces loyal to president Bashar al-Assad had been able to maintain control over Khalkhalah airport and repel the attack.

Khalkhalah lies along a major highway between Damascus and the government-held city of Sweida, a stronghold of the Druze minority that has largely avoided the bloodshed of Syria's war.

The attack on Khalkhalah was the first by IS, but the airport has been previously targeted by rebels and Syria's Al Qaeda affiliate.

In March, Syrian rebels and Islamist fighters seized the town of Bosra al-Sham, south of Sweida, on the same highway as Khalkhalah airport.

Also on Friday, Al Qaeda's Syrian affiliate withdrew from a key area along Syria's border with Jordan, the observatory said.

Rebels and the Al Nusra Front took control of the Nasib crossing in the southern province of Daraa from regime forces last month.

Nasib had been under attack by moderate rebel forces, but fell shortly after Al Nusra joined the ongoing offensive.

The jihadists and other rebels held the checkpoint, the duty-free zone between the two crossings, and the customs area.

Al Nusra withdrew from the checkpoint last week and left the other areas Friday.

"The other fighters asked them to leave Nasib because they weren't in the fight to begin with," SOHR chief Abdel Rahman said.

Syrians walk through the rubble following reported rocket attacks by rebel fighters on the Christian neighbourhood of Suleimaniyah in northern Aleppo. ( AFP: George Ourfalian )

Aleppo rocket strikes kill at least eight

Meanwhile, insurgents have bombarded a government-held part of Syria's second city Aleppo, killing at least eight people.

The SOHR said eight people were killed in an air strike by government forces in a separate, rebel-held part of the city.

Aleppo, near the Turkish border, is a major frontline in the Syrian war.

Rebel groups in and around the city have repelled repeated attempts by the Syrian army and militia fighting with it to cut supply lines from Turkey to the rebels.

State TV broadcast pictures showing heavily damaged buildings, some with their facades torn off, and streets strewn with rubble in the Suleimaniyah district of government-held Aleppo.

State media put the confirmed death toll at eight and said dozens more people were trapped under rubble.

The SOHR put the number of dead at five and said the number was likely to increase.

Syrian grand mufti Ahmed Badr al-Din al-Hassoun urged the complete destruction of insurgent-held areas from which shells were being fired.

"We inform the civilians there, be they supporters [of the insurgents] or not, to leave the area. Every area from which a shell is fired, should be completely destroyed," he said.

State news agency SANA described the insurgents behind the attack as hardline Islamist militants "linked to the Erdogan regime", referring to Turkish president Tayyip Erdogan, who wants to see Mr Assad removed from power.

Rebel-held parts of Aleppo have faced regular air strikes by the Syrian military.

A UN commission of enquiry on Syria has recorded the intense use of barrel bombs — improvised bombs dropped by helicopter — by government forces in the city.

The SOHR said dozens more people had been wounded in Saturday's air strike in the Maadi district of Aleppo and the death toll would likely increase.

More than 215,000 people have been killed in Syria's four-year war.

ABC/Wires