German Chancellor Angela Merkel said Thursday that President Bashar al-Assad should be involved in any Syrian peace talks, amid mounting pressure to end the four-year conflict.

"We have to speak with many actors, this includes Assad, but others as well," Merkel told a press conference after an EU summit on the migration crisis sparked by the Syrian war.

Like tens of thousands of their fellow countrymen fleeing war, members of a Syrian rock group exiled in Lebanon undertook a perilous journey to reach Europe.

But their trek, which has already taken them through five countries, has led to an unexpected 'exile concert tour', on the way to their hoped-for final destination of Germany.

Hungarian Prime Minister Viktor Orban on Wednesday rejected what he called German Chancellor Angela Merkel's "moral imperialism" in Europe's migrant crisis.

When asked what he expected from Merkel while Europe grapples with an influx of asylum-seekers, the hardline leader grinned: "I have a long list."

German rail operator Deutsche Bahn said Tuesday it would suspend key services to and from Austria and Hungary until October 4, citing border controls introduced to manage a record migrant influx.

The route from Budapest via Salzburg, Austria to the southern German city of Munich has been taken by tens of thousands of refugees in recent weeks.

German intelligence warned Tuesday that the number of Islamic extremists in the country had increased sharply in recent months and expressed serious concern that they were recruiting among refugees.

The Office for the Protection of the Constitution (BfV), Germany's domestic security watchdog, said the number of radical Salafists had surged to 7,900 in September from 7,500 in June, and that many were trying to lure asylum-seekers into their ranks.

Germany on Monday criticized local elections being organized by pro-Moscow separatists in eastern Ukraine as a breach of a February peace deal and urged Russia to distance itself from the idea.

"We consider with great concern the announcement by pro-Russian separatists of the organization of elections, so-called municipal elections," said Chancellor Angela Merkel's spokesman Steffen Seibert.

A growing number of "disillusioned" Islamic State fighters are defecting from the jihadist group and could be used by governments to deter potential recruits, a report published Monday said.

At least 58 people have left the group and publicly spoken about their defection since January 2014, according to the report by the International Centre for the Study of Radicalization and Political Violence (ISCR) at King's College London.

Germany's interior minister on Monday urged Europe to pick up refugees directly from crisis regions, to help them bypass traffickers who charge huge sums for dangerous journeys to the continent.

"I suggest we agree on a generous quota in which we take refugees from crisis regions to Europe without allowing people smugglers to profit from it," said Thomas de Maiziere at a press conference in Berlin, adding that he was expressing a personal view.

Berlin police have warned that there are risks of attacks by Kurdish militants against Turkish targets in Germany, according to an internal document seen by Welt am Sonntag newspaper.

The document classified "for use only by the service" said militants from the Kurdistan Workers' Party (PKK) could carry out "offenses against state institutions including Turkey's consulate and embassy as well as cultural and commercial facilities in the form of media-attention attracting occupations, vandalism and arson."

German police are investigating an asylum-seeker on suspicion that he fought for the Islamic State group in Syria, newspaper Welt am Sonntag reported Sunday.

The suspect is a Syrian national and lives in an asylum-seeker shelter in the northeast region of Brandenburg, the newspaper reported, quoting security sources.