Reports of violence and abductions among Eritrean refugees in the Tigray region of Ethiopia would be a major breach of international norms if confirmed, the UN said on Sunday.
Aid groups have not been able to enter the area freely, making it difficult to verify the claims.
"We have heard reports, that are not confirmed, of violence and abductions. I hope these are not true because they would be major violations of international norms and standards in the treatment of refugees and there's very little we can do without having access," UN high refugee commissioner Filippo Grandi said from Khartoum.
“So my strong appeal to the prime minister of Ethiopia is for the situation to be addressed as a matter of urgency.”
Mr Grandi said he hoped humanitarian groups would be allowed into Tigray "in the next few days".
Ethiopia hosts about 100,000 Eritrean refugees and has done so “in a very generous way”, the UN said.
Doctors Without Borders (MSF) said refugees who initially fled the conflict in Tigray and entered Sudan had returned to Tigray, including a minority of ethnic Eritreans.
The returning refugees crossed into Ethiopia to help more family members return with them into Sudan, or to bring back more belongings, MSF said.
"Some are going back to sell their assets and return to Sudan with money. A small number of the people arriving are originally from Eritrea and they were previously displaced into Ethiopia, before now fleeing again to Sudan."
Armed individuals are also crossing into Sudan. They are being told to hand their weapons over and are separated from the refugee camps to keep these places "civilian in character", Mr Grandi said.
About 44,000 people have fled from Ethiopia into Sudan and the numbers increase every day.
The UN made an appeal to donors for $147 million to provide humanitarian aid for the refugees.