German Chancellor Angela Merkel was scheduled to travel to Istanbul on Sunday desperate to secure Turkey's help in stemming the flow of migrants from the Middle East to Europe, but without being seen to sacrifice their human rights.

Dubbed a "punch-bag" for her own party by German media due to frustrations over the crisis, Merkel wants to cement a European deal with Turkey on aid and closer ties in return for help in encouraging refugees there to stay put.

Merkel has resisted pressure to tighten Germany's border controls and turn away refugees arriving from Austria, even as Germany expects 800,000 to 1 million new arrivals this year.