About 75,000 refugees and migrants stranded in several European countries, among them some 24,600 children, are at risk of psychosocial distress due to living in a prolonged state of limbo, the United Nations Children's Fund warned on Thursday.

Despite having the right to reunite with family members in Western European countries, the majority of asylum seekers in Greece, Bulgaria, Hungary and the Western Balkans did not know if and when they would be able to move on with their journeys, UNICEF said in a statement.

"The family reunification process is slow, and its outcome uncertain, and it is this uncertainty which can cause significant emotional distress and anxiety for children and families, setting them back for years to come," said UNICEF Regional Director and Special Coordinator for the Refugee and Migrant Crisis in Europe, Afshan Khan.

According to the agency, the "situation is particularly acute for single mothers and children" stranded in Greece or the Balkans while waiting to join relatives in other European Union member states.

In many cases, male adults are the first to make the journey to Europe; the other family members following behind later.

However, due to border closures in 2016 and an EU-Turkey deal on the return of migrants, some were being kept in transit countries from where they have to apply for family reunification, UNICEF said.

Of about 5,000 family reunification requests submitted in Greece in 2016 -- 700 of these made by minors who were either unaccompanied or separated from their families -- only 1,107 people had arrived at a final destination by the end of the year, according to UNICEF.

"With the number of those stranded continuing to rise, it is incumbent on member states to alleviate procedural bottlenecks so that families can get back together as quickly as possible," Khan added.