Nearly 50,000 people, most of them women and children, are stranded at Syria's southern border with Jordan, an increasingly unsafe area where air attacks were reported over the past few days, the United Nations said on Monday.

UN deputy spokesman Farhan Haq told reporters that "some people are reportedly attempting to leave the area, risking further danger and deprivation in an inhospitable desert location".

Those remaining in the area, known as a berm, face a scarcity of food and healthcare, Haq said. In one section, called Hadalat, an estimated 4,000 people are reportedly living solely on flour and water, he said.

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On Thursday, Syrian government forces and their allies captured a key area along the Jordan border in their latest push against rebel groups there.

Syrian state media said the capture closed major smuggling points used by rebels to bring weapons and fighters from Jordan into the war-torn country. Syrian state TV said government troops captured an area of 1,300 square kilometres in addition to some strategic hills.

Jordan closed its border with Syria in June 2016 after a car bomb attack by the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group staged from near Rukban killed seven Jordanian border guards. Since then, international aid organisations have wrestled with the dilemma posed by sending aid to an off-limits area.

UN agencies agreed late last year to an aid system that critics say handed much of the control over aid distribution to Jordan's military and a Jordanian contractor and also involved armed men on the Syrian side.

Haq said on Monday that UN agencies "are deeply concerned about the security and protection" of the nearly 50,000 people stranded at the border.

"The UN calls on all parties to the conflict to take the necessary steps to prevent further harm to the frightened and highly vulnerable individuals stranded at the border," he said.

UN agencies are ready to continue supporting Jordanian authorities, despite limited resources, and "immediately provide protection and additional life-saving assistance as needed," Haq said.