d Ra'ad Al Hussein warned in a statement of the "grave risk that the intensified fighting will see many civilians trapped".

The bombardment has already forced more than 66,000 to flee their homes in search of safety, according to the UN, while others huddle in their basements to wait out the raids.

With Russia's help, Syrian President Bashar al-Assad's army has battered Daraa province for over a week with air strikes, rocket fire and crude barrel bombs.

Civilians fleeing attacks on rebel-held towns in southern Syria are being used as "pawns", the UN rights chief said, lamenting reported demands for payment at government checkpoints and jihadist's blocking movement.

He said many risked being caught between pro-government forces on one side and armed opposition groups and the Islamic State jihadists on the other.

The rights chief condemned how "civilians in Syria continue to be used as pawns by the various parties".

Zeid said his office had received reports that "in the last few days, civilians at some government checkpoints in the southern-eastern and western parts of Daraa have only been allowed through to government-held areas in Daraa City and As Suwayda governorate for a fee."

"To add to the bleak situation facing civilians, there are also reports that ISIL fighters in control of the Yarmuk Basin area in the western part of Daraa governorate are not allowing civilians to leave the areas under their control", he said.

Zeid stressed that international law requires all sides to "do their utmost to protect civilians" and urged the parties to the conflict "to provide safe passage to those wishing to flee."

"Those wishing to stay must be protected at all times," he added.

Zeid said his office had documented at least 46 civilian deaths in the region since the escalation began on June 19. But the Syrian Observatory for Human Rights puts the toll at more than double that, at 96.

The UN has warned that more than 750,000 lives are at risk in the south, which is meant to be protected by a ceasefire put in place last year by Russia, Jordan, and the United States.

The onslaught has sparked fears of a re-run of the offensives last year against the rebel enclaves of Aleppo and eastern Ghouta, including deadly bombardments followed by a retaking of territory and an accord to evacuate rebels from the areas.

"I have spoken of the cruel irony of Eastern Ghouta being a de-escalation zone, and how the conduct of the war has been utterly shameful from the outset and a stain on us all," Zeid said Friday.

"Now another supposed 'de-escalation' zone risks becoming the scene of large-scale civilian casualties," he said.

"This madness must end."