An alliance of Syrian rebel forces seized a key army base in the south of the country on Tuesday in a new setback for the regime's embattled troops.

The Southern Front alliance took full control of the 52nd Brigade base in Daraa province after 24 hours of fierce clashes, a spokesman for the group told AFP.

"The 52nd Brigade base was fully liberated from the regime army," Major Essam al-Rayes said, adding that at least 2,000 rebel forces had participated in the "short and quick" assault.

The base lies near a major highway running from Damascus to Syria's southern border with Jordan and is also near the border with neighbouring Sweida province, which is largely regime-controlled.

"This base was one of the main lines of defence for the regime forces. It was a nightmare, because they used it to shell all the areas to the east of the province," al-Rayes said.

"The base is also an important infantry base, from which the regime attacked towns and villages in the south," said Diaa al-Hariri, spokesman for Faylaq al-Awwal, one of the rebel groups in the Southern Front coalition.

The Syrian Observatory for Human Rights monitoring group confirmed that opposition groups had taken the 52nd Brigade base after clashes and intense shelling that left 14 rebel fighters and 20 government forces dead.

Observatory chief Rami Abdel Rahman said regime troops withdrew to the nearby village of al-Dara.

Rebel groups control a majority of Daraa province and its capital, Abdel Rahman said.

Syria's official news agency SANA did not report the capture of the base.

But earlier, citing a military source, it said the air force had struck the area, killing at least 40 "terrorists," who it accused neighbouring Jordan of backing.

The fall of the base is the latest in a string of defeats for the Syrian regime, which has lost territory to both rebel alliances and the jihadist Islamic State group.

Government troops have been all but driven from northwestern Idlib province in recent weeks by a coalition including Al-Qaeda affiliate Al-Nusra Front and other largely Islamist rebel forces.

It has lost the city of Palmyra and the surrounding region to the Islamic State group, which now controls around half of Syria's territory.

The loss of Brigade 52 also follows earlier defeats for the regime in Daraa, where it last month lost control of the Nasib border post, its last crossing with Jordan.

Analysts say the regime has decided to concede the de facto partition of the country, and will focus its depleted military forces on defending territory it considers strategically important.

They say the regime will focus on Damascus, the coastal provinces of Latakia and Tartus, and the central cities of Homs and Hama.

Syria's conflict began in March 2011 with anti-government protests.

But after a regime crackdown, it spiralled into a civil war that has killed more than 230,000 people, according to a new toll issued by the Observatory on Tuesday.

The Britain-based group said nearly 70,000 civilians have been killed in the war, nearly 11,500 of them children.

More than 85,500 government forces -- soldiers and militiamen -- have been killed, along with over 41,000 rebels, Syrian jihadists and Kurdish fighters, the monitor said.

It also documented the deaths of 31,247 foreign jihadists, and said another 3,191 people killed in the conflict have yet to be identified.

Many of the civilians deaths have come in government aerial attacks, particularly involving the use of crude "barrel bombs".

The weapon, criticised by rights groups as indiscriminate, has been used to devastating effect in Aleppo, where the Observatory said four members of one family were killed in a barrel bomb attack on Tuesday.

Syria's conflict has evolved into a complex, multi-front war that has drawn in jihadists including IS, which now rules a self-proclaimed "caliphate" in territory across Syria and Iraq.

The group on Tuesday claimed an attack against a local government headquarters in Amriyat al-Fallujah, west of the capital Baghdad.

At least two people were killed in the assault by militants armed with assault rifles, pistols and suicide belts.

The attack came a year to the day since IS launched a sweeping offensive that overran much of Iraq's Sunni Arab heartland.