German Chancellor Angela Merkel on Friday called on Russian President Vladimir Putin to accept the OSCE's assessment on Ukrainian presidential elections this weekend.

"I expect Russia to respect the doubtlessly objective assessment of the OSCE -- after all it is a member of the organisation," Merkel told newspaper the Saarbruecker Zeitung.

Turkish Prime Minister Recep Tayyip Erdogan's planned pre-election address in Germany Saturday has sparked widespread criticism in the wake of the country's mine disaster and concern over opponents' protest calls.

Amid heightened anger within Turkey over his response to last week's tragedy in which more than 300 died, Erdogan is set for a long-scheduled appearance before supporters in the western city of Cologne.

A German parliamentary panel looking into U.S. Internet and telecoms surveillance plans to invite the heads of Facebook, Google, Apple and Microsoft, a lawmaker said Wednesday.

A motion to invite them as witnesses would be adopted at a Thursday meeting of the committee of inquiry, Social Democratic panel member Christian Flisek told reporters.

Pro-Russia protesters disrupted a public meeting with German Chancellor Angela Merkel in Berlin Wednesday, calling for the arrest of "Nazis" in Ukraine.

Around a hundred protesters, wearing caps emblazoned with the Russian two-headed eagle emblem, booed the chancellor and brandished placards bearing slogans such as "Arrest the Nazis in Ukraine" and "No war against Russia".

Yemeni tribesmen have freed a German man who was kidnapped in the capital Sanaa in late January, state news agency Saba reported on Tuesday.

Rudiger Schwidt, who is in his 60s, was abducted by a tribesman pressing authorities to release his two detained sons, according to a foreign ministry official.

German Foreign Minister Frank-Walter Steinmeier said Tuesday that Ukraine's May 25 presidential vote will be crucial in bringing the country out of crisis, after he arrived in Kiev on a key visit.

"We all agree that the presidential election on May 25 will play a crucial role," Steinmeier told reporters after talks with Ukrainian officials, adding that he hoped steps in Ukraine will "bring back occupied territory, disarm armed groups step-by-step and reinstall the authority of the state."

Germany this week launches another attempt to mediate in the Ukraine crisis, pushing for dialogue between the pro-Western government and pro-Russian groups to resolve the festering and violent conflict.

Foreign Minister Frank-Walter Steinmeier will visit Ukraine on Tuesday to support a "national dialogue" between Kiev and the pro-Russian regions in the east, said his spokesman in Berlin.

The foreign ministry said Monday that France and Germany intend to prevent Syrians living in their countries from voting in Syria's presidential election, expected to return President Bashar Assad to power.

Germany and France are "preventing Syrians living in their territory from voting", the foreign ministry said.

The United Nations' atomic watchdog said Sunday it would hold talks with Iran to discuss its long-stalled probe into Tehran's alleged past efforts to develop nuclear weapons.

"IAEA and Iranian officials will meet on Monday in Vienna for further discussions within the Framework for Cooperation," International Atomic Energy Agency spokeswoman Gill Tudor said.

EU heavyweights France and Germany warned on Saturday that it could be weeks after May 25 elections before a new head to the European Commission is chosen by bloc leaders.

"There is a meeting of European leaders and heads of government set for May 27, but the decision will not be made on the Tuesday or Wednesday following the elections," said German Chancellor Angela Merkel during a visit by French President Francois Hollande to northern Germany.