Amnesty International has accused the European Union of turning a blind eye to Croatian police violence targeting migrants and called for a probe into alleged abuses.
Thousands of migrants from Asia, the Middle East and Africa go through the Balkans every year in a bid to enter European Union member state Croatia and then move to western Europe.
While most manage to continue their journey, several thousand were thwarted last year by Croatian border police and are now stranded in migrant centres in neighbouring Bosnia.
Amnesty International cited an incident which allegedly took place in late May near the Bosnian border.
A group of 16 Pakistani and Afghan asylum-seekers were "bound, brutally beaten and tortured" by Croatian police after having illegally entered the country, the human rights monitor said in a statement.
An injured migrant. Source: Danish Refugee Council/Amnesty International
Police also "smeared food on their bleeding heads to humiliate them", it said.
Amnesty cited Bosnian doctors and some of the migrants who said they were beaten with metal sticks, batons and pistols before being sent back to Bosnia.
"EU can no longer remain silent and wilfully ignore the violence and abuses by Croatian police on its external borders," it said.
"Their silence is allowing, and even encouraging, the perpetrators of this abuse to continue without consequences", Amnesty said, calling for Brussels to investigate what it called "horrifying police violence".
Croatia's interior ministry rejected the allegations and the notion "that a Croatian police officer would do such a thing or have a motive for it".
It said that on 28 May, when the alleged incident took place, a fight between migrants took place in Bosnia, which left two migrants dead and many injured.
Britain's Guardian newspaper reported another incident in which Croatian police allegedly spray-painted the heads of asylum seekers with crosses.
The Croatian government denied the allegation.
