An alliance of rebels in southern Syria announced a major offensive on Wednesday to capture remaining positions held by the Syrian military in Quneitra province, near the Israeli-occupied Golan Heights.

Quneitra sits in a sensitive region around 70 kilometres (40 miles) southwest of the capital Damascus and has been the scene of frequent fighting between various insurgent groups and the Syrian military backed by allied militia.

Rebel spokesman Issam al-Rayes wrote on Twitter that insurgent groups fighting in the operation had signed a pact, which did not involve al Qaeda's Syria wing Nusra Front.

The groups in the offensive are fighting under the banner of the rebel Free Syrian Army, he said. Nusra has fought in southern Syria but is not thought to be the main insurgent force there, unlike in other parts of the country.

A Reuters photographer watching from the Israeli-occupied Golan said there had been heavy shelling since early Wednesday in the Quneitra area, with scores of bombardments.

At one point he could see smoke rising from around 13 bombardments and the sound of small arms being fired could also be heard in the distance.

The shelling appeared to be focused between Quneitra's water reservoir and the town itself, with some buildings on the outskirts appearing to have suffered damage, the photographer said. It was not clear which groups were taking part.

The Britain-based Syrian Observatory for Human Rights monitoring group reported violent clashes between the Syrian military and insurgent groups including Islamist factions in northern Quneitra.

The Observatory's head Rami Abdulrahman said a handful of combatants were killed on both sides since Tuesday in the area.

Insurgents fighting in Sweida province further east had failed during recent fighting to capture a main road to Damascus, and it was not clear whether they could secure a route to the capital in this latest offensive, he said.

Different groups, including the hardline Islamic State and Nusra Front, have been putting the Syrian military under heavy pressure in various parts of the country in the past two months.

Another insurgent alliance including Nusra Front has taken hold of the northwestern Idlib province, edging closer to President Bashar al-Assad's coastal stronghold while Islamic State fighters overran the central city of Palmyra last month.

The government says it can defend important stretches of territory in Syria's populous west and the deputy foreign minister told Reuters last week that Damascus was safer than towards the start of the conflict, which grew out of protests against Assad in 2011.