A quarter of a million Syrian refugees could return home next year, despite massive hurdles facing returnees, the UN has said, urging support to the millions still in neighbouring countries.

"We are forecasting ... up to 250,000 Syrians go back in 2019," Amin Awad, who heads the UN refugee agency's Middle East and North Africa operations, told reporters in Geneva.

"That figure can go up and down according to the pace with which we are ... removing the obstacles to return," he stressed.

There are currently 5.6 million Syrian refugees living in the region, including around one million born into displacement, according to UNHCR data.

The agency said that 117,000 refugees had returned to Syria since 2015, including 37,000 this year.

"These are organised returns, completely voluntary, in safety ... and of course with UNHCR involvement," Awad said.

Despite the winding down of Syria's devastating conflict, which has killed more than 360,000 people since 2011, Awad said returnees face a whole host of obstacles.

These range from documentation confirming identity and property in Syria to a dire lack of education, healthcare and sanitation in the places to which they return.

There are issues related to conscription and questions around those who deserted the army when they left Syria, Awad said.

There are also obstacles related to physical security- even in places where the fighting has stopped- including large amounts of unexploded ordinance.

UNHCR is working with the Syrian government to try to improve conditions for those who wish to return, Awad said.

The UN refugee agency, along with the development agency, meanwhile launched an appeal for $5.5 billion to support national efforts in 2019 and 2020 in Turkey, Lebanon, Jordan, Egypt and Iraq to continue hosting the millions of Syrians still not ready or able to return home.

"It is critical that the international community continues to recognise the plight of Syrian refugees and provides vital support to host governments ... to help shoulder this massive burden," Awad said.

Tuesday's appeal is to help 5.6 million Syrian refugees as well as 3.9 million members of vulnerable host communities.

"Communities in the region hosting refugees from Syria have shown tremendous generosity, yet are increasingly themselves under strain," Mourad Wahba, head of UNDP's Arab States region, said in a statement.