German Chancellor Angela Merkel on Thursday urged Turkish President Recep Tayyip Erdogan to ensure full freedom of expression in Turkey ahead of a crucial referendum on constitutional change.

Making her first visit to Turkey since July's failed coup, Merkel was seeking to keep a key partnership alive after a series of crises in recent months.

Her visit -- which has caused controversy at home and in Turkey -- comes as the country prepares for a referendum expected to be held in April on constitutional changes to give Erdogan greater powers.

"I emphasised from my side that in this far-reaching change the separation of powers and freedom of expression must be ensured," Merkel said after talks with Erdogan at his presidential palace in Ankara.

Merkel added she had also passed on concerns over press freedom and the accreditation of German journalists in Turkey, several of whom have not received approval to work in 2017.

"We spoke in detail over freedoms for journalists," she said.

Merkel said the referendum on constitutional change could be monitored by a delegation from the Organisation for Security and Cooperation in Europe (OSCE) "so that what the people want is guaranteed".

Erdogan responded that a separation of powers would remain in place under the new constitution, with other centres of power along with the presidency.

"The legislative is still there. So is the executive and the judiciary. Their dissolution... is out of the question," said the Turkish president.

The relationship between the two NATO allies has been battered in recent months by a series of rows in the wake of the July 15 failed coup aimed at ousting Erdogan.

Berlin has repeatedly expressed unease over the extent of the crackdown that has seen some 43,000 people arrested in the wake of the putsch, under a state of emergency that has now lasted over half a year.

Erdogan, meanwhile, has vented his exasperation that Germany has not responded to requests for the extradition of hundreds of suspects linked to the coup, the Kurdish militant movement and the ultra-left.

Last week it was reported that 40 Turkish soldiers stationed at NATO bases had asked Berlin for asylum, with Turkey pressing for the bids to be rejected.

Merkel emphasised that Germany was Turkey's partner in the fight against terror but could only act against suspects "when we have evidence and it is examined by the courts".

Erdogan said he hoped Berlin would take "more rapid decisions" on the issue.

The controversy over Ankara's post-coup crackdown has dealt a new blow to its long-running EU membership bid, although Berlin has stopped short of backing Austrian calls for the entire process to be halted.

Merkel also wants Turkey to keep implementing a deal that has so far successfully reduced migrant flows to Europe, despite threats by Erdogan to walk away from it due to a failure to fulfil a pledge to grant Turks visa-free travel.

"The refugee issue is very important for Turkey and also the EU and Germany," she said, adding the deal was in their "mutual interest".

With the hugely controversial constitutional referendum looming, the visit has not been universally welcomed in Turkey, with opposition Republican People's Party (CHP) leader Kemal Kilicdaroglu saying he was disappointed Merkel was not emphasising more that "Turkey must improve its democratic standards".

But Merkel was due to meet Kilicdaroglu and figures from the pro-Kurdish People's Democratic Party (HDP) whose co-leaders are under arrest.

Merkel said after meeting Erdogan: "Opposition belongs to a democratic state."

Opposition figures in Germany have also suggested Merkel should not be meeting Erdogan as he is presiding over the crackdown and Turkey policy is set to be a key issue when Merkel faces re-election in September.

Germany is home to some three million people of Turkish origin, the biggest population of Turks in the world outside Turkey.

Interior Minister Thomas de Maiziere defended the trip in comments to the Passauer Neue Presse, saying: "The chancellor does not need advice. She knows what she has to do."