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

Dubbed a "punch-bag" for her own party by some German media due to frustrations over the refugee 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.

By going to Istanbul just two weeks before elections in Turkey, Merkel risks Turkish President Tayyip Erdogan drawing her into his party's election campaign amid growing European concerns about freedom of speech in Turkey's fragile democracy.

"She has manoeuvred herself into such a 'no win' situation," said Ekin Deligoez, a Turkish-born German lawmaker with the opposition Greens.

"And in such a situation, Mrs Merkel comes along with the wish for friendship! Erdogan knows that and he is using it," Deligoez said, one of 3.5 million people living inGermany who are Turkish nationals or of Turkish origin.

Just two months ago, Merkel was practically able to dictate terms to Greece over an aid plan to tackle its debt crisis. But over neighbouring Turkey, she has far less leverage to get her way, particularly as just 10 days ago, Merkel reiterated her opposition to Turkey joining the European Union.

That comment came before an EU offer to Turkey last week of an "action plan" including "re-energised" talks on joining the bloc, as well as aid and the prospect of easier travel visas, in return for its help stemming the migrant flow, a problem Merkel has called a test of historic proportions.

Turkish Foreign Minister Feridun Sinirlioglu said on Friday the plan had not taken final shape and it was wrong to give the impression that Turkey wanted a certain amount of funds to keep refugees in Turkey.

PRINCIPLES AND POLITICS

Erdogan and his ruling AK Party, which faces a general election on November 1, have an interest in avoiding any suggestion of a sellout to help ungrateful Europeans.

He is using the opportunity to grandstand and maximise his leverage over Europe, and Merkel. On Friday, the president accused the EU of insincerity in talks on Turkish membership.

In truth, both Erdogan and Merkel know there is no near-term prospect of Turkey joining the EU. But both can still gain something from Sunday's meeting.

For Merkel, nailing down Turkey's commitment to the action plan is important to stemming the refugee flow and relieving the political pressure on her at home, including from her own party.

For Erdogan, winning the promise of an accelerated path to visa-free travel to the EU for Turks could be an election boon. Inclusion on a list of "safe countries" could also help Turkish tourism.

A German official said on Wednesday that Berlin was willing to support an EU proposal to put Turkey on the list, an idea it has previously been sceptical of, mainly because of Ankara's human rights record and its treatment of the Kurdish minority.

But Merkel is under pressure not to sacrifice human rights to European realpolitik. Amnesty International is critical of Ankara's treatment of refugees.

"Merkel must put principles before politics in her talks with the Turkish government," said Amnesty's Andrew Gardner.