Turkey on Saturday called in Germany's ambassador to Ankara as tensions have flared between the two countries following Turkey's violent crackdown on protesters.

Berlin and Ankara on Friday both summoned each other's ambassadors in tit-for-tat moves as fresh obstacles have also emerged over the past week linked to Turkey's bid to join the European Union.

Amid a sharp spike in tensions between Turkey and the EU, Berlin and Ankara summoned each other's ambassadors in tit-for-tat moves Friday as fresh obstacles to the Turkish bid to join the bloc emerged.

German Foreign Minister Guido Westerwelle called the envoy to his offices "due to remarks from Turkish officials toward Germany", a ministry spokesman said, leading Ankara to later threaten consequences over the row.

Germany summoned the Turkish ambassador Friday over sharp criticism from Ankara over reluctance to move forward with Turkey's EU bid, a foreign ministry spokesman in Berlin said.

Turkish European Union Affairs Minister Egemen Bagis told reporters Thursday that Germany's resistance to opening a new chapter in EU accession talks was linked to Chancellor Angela Merkel's "election campaign".

Germany is in talks with Ukraine on an offer to treat jailed former prime minister Yulia Tymoshenko for her health problems in Berlin, Foreign Minister Guido Westerwelle said Friday.

"The German offer of medical treatment for Mrs Tymoshenko in Berlin remains in place. I emphasized this," Westerwelle told reporters after meeting Ukrainian President Viktor Yanukovych in Kiev.

U.S. First Lady Michelle Obama and daughters Malia and Sasha on Wednesday visited key sites of Germany's troubled Nazi and Cold War history, walking through the haunting Holocaust Memorial and laying flowers for those killed at the Berlin Wall.

As President Barack Obama attended to matters of state during a 24-hour visit to the German capital, his family went on a whirlwind tour in an armored limousine on their first visit to the city, where 8,000 police secured the presidential visit.

President Barack Obama on Wednesday defended U.S. Internet and phone surveillance programs which sparked alarm in Europe, saying American spies were not "rifling" through the emails of German and French citizens.

Obama insisted that the programs run by the super secret U.S. eavesdropping service the National Security Agency (NSA) were legal and limited and were a vital tool in the fight against global terrorism.

Barack Obama arrived for his first visit as U.S. president to Berlin on Tuesday for talks with German Chancellor Angela Merkel and a major open-air speech at the city's Brandenburg Gate.

Obama arrived at Berlin's Tegel Airport from Northern Ireland where he and Merkel took part in a G8 summit dominated by the bloodshed in Syria and a bid by the European Union and the United States to create the world's biggest free trade area.

Germany said Monday there was an "urgent" need to find a solution over the Iranian nuclear program, calling for continued dialogue with President-elect Hassan Rowhani, a moderate cleric.

"A solution to this issue is urgent," foreign ministry spokesman Andreas Peschke told reporters.

German Chancellor Angela Merkel said Monday that she was shocked by Turkey's violent crackdown on protests in major cities, saying it "was much too harsh".

"There were horrible images in which one could see that the approach was much too harsh in my view," Merkel told commercial broadcaster RTL in reaction to news coverage of riot police breaking up demonstrations in Istanbul and the capital Ankara.

Germany on Sunday urged Turkey to "respect the freedom to demonstrate and freedom of expression," saying that Ankara was sending the wrong signal by cracking down on protesters.

"The German government has warned several times: freedom to demonstrate and freedom of expression must be guaranteed and peaceful citizens respected," government spokesman Steffen Seibert said in a tweet, reacting to the latest violence in Turkey.