More aid agencies helping migrants arriving in Greece say they are joining a boycott of detention centers.
Human rights organizations have criticized a deal between the European Union and Turkey to fast-track registration and asylum applications. Migrants whose applications fail will be sent back to Turkey.
Aid agencies said cooperating with the Greeks at detention centers would make them complicit with an "unfair and inhumane" practice.
On March 22, the UN refugee agency UNHCR and aid organization Doctors Without Borders both announced they would cut back assistance.
A day later, the International Rescue Committee and Norwegian Refugee Council joined them.
There are almost 50,000 migrants stranded in Greece, the vast majority of them not detained in camps since most arrived before the new EU arrangement came into effect on March 20.
More than 147,000 people, many fleeing conflict in the Middle East and Asia, have arrived in Greece by sea this year.