An estimated 24,600 refugee children are "living in limbo" waiting to join family in Western Europe, UNICEF warned in a report released on Thursday.

Single mothers and children are among nearly 75,000 refugees and migrants stranded in Greece, Bulgaria, Hungary and the Western Balkans as they wait to find out when and whether they can join male relatives who have gone on ahead to EU countries, the NGO said in a statement.

"We are seeing single mothers and children stranded in Greece, Serbia and Bulgaria who have not seen their husbands and fathers for months or even years," said Afshan Khan, UNICEF Regional Director and Special Coordinator for the Refugee and Migrant Crisis in Europe.

"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."

According to UNICEF, reunification requests can take between ten months and two years to be processed and E.U. countries have varying policies on whether to allow asylum seekers to join family.

Nearly 5,000 family reunification requests were made from Greece in 2016, 700 of which were from unaccompanied and separated children. 1,107 successful applicants reached their destination country by the end of the year. The European Union rules say it must take in unaccompanied children who have family ties to European countries under so-called Dublin rules. Under the so-called Dubs amendment passed by parliament last year, Britain agreed to accept vulnerable refugee children who arrived in the European Union before March 20, 2016. But in February, the government announced plans to drop the scheme it said encouraged human trafficking. Charities working with refugees welcomed the announcement but expressed concern about the thousands more lone migrant children in Europe who will not benefit from the scheme. Around 30,000 unaccompanied children arrived in Greece and Italy alone in 2016, according to UNICEF.