Thousands of Syrian refugees are stranded in the desert as Jordan has blocked their entry, aid agencies have said.

Some 12,000 people are facing rapidly deteriorating humanitarian conditions, the UN refugee agency said yesterday, and the vast majority are women and children. The number has skyrocketed from 4,000 a few weeks ago after an exodus partly triggered by Russian airstrikes on rebel-held areas.

"We appeal to the government of Jordan to allow refugees stranded at the border to enter the country," UNHCR spokeswoman Melissa Fleming said at a briefing in Geneva.

"If refugees are not admitted to Jordan and substantial assistance not provided, the lives of refugees will be at risk in the coming winter months."

Since the beginning of 2015, the system for processing refugees on the Syrian-Jordanian border has slowed significantly, with only a few dozen each day allowed to move into the transit zone. According to local sources there is no system for identifying the most vulnerable and processing them first.

Human Rights Watch (HRW) heavily criticised the Jordanian government and called on them to "stop stranding people in remote border areas for months on end."

"Jordan is blocking 12,000 people fleeing war in remote desert areas without proper help," said Nadim Houry, HRW's deputy Middle East and North Africa director.

"The authorities should swiftly allow them to reach transit centres and should make sure they get all the help they need."

As the numbers swell, aid agencies report conditions worsening among those on the border and children falling prey to diarrhoea, vomiting and acute malnutrition.

As well as refusing entry to thousands of refugees, Jordan has also been accused of retuning refugees to their war-torn homeland, which is illegal under international law.

Jordan is one of the countries worst affected by the refugee crisis and currently hosts over 630,000 refugees.