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on Friday urged the Ìýto immediately allow humanitarian aid into a rebel holdout under regime siegeÌýin the southern .

The has warned of food shortages in opposition-held Daraa Al-Balad, which is surrounded by Russia-backed Assad regime fighters seeking to retake control of the area.

Clashes have raged between both sides in recent weeks, before a shaky Moscow-brokered truce deal saw dozens of rebels quit the area this week.

"The Syrian government must immediately lift the siege to facilitate unfettered access for humanitarian organisations and allow the medical evacuation of the sick and injured," said Amnesty's Syria researcher Diana Semaan.

She called on all sides to "grant safe passage to civilians wanting to flee the area".

The rights group said the government now rarely approved medical evacuations, while many of the ill or wounded feared they would be detained or face reprisals if they stepped into regime-held territory.

One woman told Amnesty last week the shops were almost empty of all food.

Her cousin had died because he was not granted permission to leave for urgent medical treatment, she told the rights group.

The UN humanitarian agency OCHA said on Tuesday that 38,600 people -- more than half of them children -Ìýhad fled Daraa Al-Balad and been registered in and around the city.

Amnesty said the exodus happened after the regime briefly opened a checkpoint for people to leave the area.

Around 20,000 people remained inside with scarce supplies, it said.

The Syrian Observatory for Human Rights, a Britain-based war monitor with sources inside Syria, said regime forces fired mortar rounds at Daraa Al-Balad on Friday.

There was no sign of any resumption of the evacuation of rebels under the truce deal, it added.

The Observatory had said that 53 people, mostly "fighters who rejected the reconciliation deal", left on Thursday.

A Russian-brokered truce aimed at ending the region's worst fighting in years resulted in a a second group of rebel fighters leaving the Syrian city of

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The Syrian regime's news agency said the number had been 45 and included family members.

Thursday's evacuation came two days after an initial smaller group of opposition fighters boarded buses bound for the rebel-held north of the country, according to the Observatory.

The wider province of Daraa, seen as the birthplace of Syria's uprising in 2011 and held for years by opposition forces, was .

Some rebels left under a previous Moscow-backed ceasefire at the time, but others kept their weapons and remained of several areas, including Daraa Al-Balad.

Daraa Al-Balad has been besieged sinceÌý, following the locals' refusalÌýto accept the country'sÌýsham presidential vote, which again returned dictator Bashar Al-Assad as leader.