When President Barack Obama and other world leaders gather in Germany next week, Russia's Vladimir Putin will be left off the guest list, part of his punishment for more than a year of alleged Kremlin-supported aggression in Ukraine.

But despite vows from Obama and his European counterparts to isolate Putin as long as the crisis in Ukraine remains unresolved, the Russian leader is still a central player in major international affairs, including the U.S.-led nuclear talks with Iran.

Germany's foreign minister warned on Friday that the situation in Ukraine's separatist east was turning more "fragile" and that negotiators must focus their efforts on averting further deterioration.

Frank-Walter Steinmeier flew into Kiev just days before Ukraine's Western-backed President Petro Poroshenko's personal envoy meets pro-Russian negotiators in the Belarussian capital Minsk for talks aimed at salvaging a February ceasefire agreement.

Chancellor Angela Merkel vowed Friday Germany would play a "constructive" role to help keep Britain in the EU and work with Prime Minister David Cameron to reform the 28-nation bloc.

Offering a warm and conciliatory tone to visiting Cameron -- who had received mixed welcomes elsewhere on a two-day European whistlestop tour -- she said Berlin was open to a compromise on London's demands.

A German aid worker kidnapped in Afghanistan in April is free and safe, the German foreign ministry said on Friday.

"Foreign minister (Frank-Walter) Steinmeier is relieved that a German man who was abducted in mid-April has been free since last night," the spokeswoman told reporters.

Belgium has launched an investigation into claims of widespread espionage by Germany, which is accused of helping the U.S. spy on Berlin's closest allies in Europe, a minister said Friday.

"If it should emerge that the reports of wide-scale eavesdropping by the German secret services are correct, Germany will have to provide an explanation," Telecoms Minister Alexander de Croo said on Friday.

The EU referred Germany to court Thursday for failing to regularly monitor security measures at some airports, but said it was not related to the Germanwings crash.

Germany failed to act on repeated requests from Brussels to comply with EU law on airport security and the case will now go to the European Court of Justice, the European Commission said.

German Chancellor Angela Merkel and France's Francois Hollande will likely meet next week to finalize plans for a more integrated eurozone as the threat of a British exit from the EU looms, European sources said.

The Berlin meeting, which will also include European Commission head Jean-Claude Juncker, was "almost certain", the sources said, in a move that will cause tremors in a euroskeptic London suspicious of a two-speed Europe dividing euro and non-euro nations.

A German deputy with strong links to Ukraine was denied access to Russian territory over the weekend, a decision slammed as "unacceptable" by Berlin on Monday.

The German foreign ministry said the government had made an official protest through the Russian embassy in Berlin.

The head of the U.N. refugee agency appealed on Saturday to the international community to boost development aid countries hosting Syrian refugees such as Jordan and Lebanon.

"They are the first line of defense for global collective security and they are pillars, essential pillars, for regional security," Antonio Guterres told The Associated Press at a regional World Economic Forum conference held in Jordan.

German Chancellor Angela Merkel said Friday there was still a lot of work needed to reach agreement with Greece on its debt bailout, as tortuous talks drag on amid fears Athens could run out of money.

The government of Greek leftist Prime Minister Alexis Tsipras is locked in talks to obtain fresh funding with international creditors who are demanding more tough austerity measures in return.