A bus that carried 31 Syrian refugees to German Chancellor Angela Merkel's office in protest returned Friday to the small town that organized the road trip widely condemned as a political stunt.

The Bavarian district chief behind Thursday's journey to Berlin, Peter Dreier, had called it an "act of desperation" as his southern rural area buckled under the strain of a mass influx that brought 1.1 million migrants to Germany last year.

German prosecutors said Wednesday they had received several criminal complaints against the suspended police chief of Cologne and officers for failing to prevent a spate of violence against women blamed on migrants.

"Several criminal complaints were filed against the former police chief of Cologne and other police officers in connection with the events at Cologne main rail station," said the state prosecutor for the city of Aachen.

German prosecutors said Wednesday they have charged four people with forming a right-wing extremist "terror" group which was allegedly planning a bomb attack at an inhabited refugee shelter.

The four German suspects are accused of starting a "far-right terrorist organization" called Oldschool Society with a larger group of people in 2014.

Ten Germans were killed in the suicide bombing that ripped through the historic heart of Istanbul, Germany's foreign ministry said Wednesday.

"I must relay the sad news that we now have 10 dead Germans among the victims" from Tuesday's attack, ministry spokeswoman Sawsan Chebli told reporters.

Germany Tuesday admitted that it was refusing entry to an increasing number of migrants seeking to cross over from Austria, but said it was not a result of any policy changes.

"It is true that the number is higher in the past days," said a federal police spokesman, adding however that the number of people turned away at the border was in the "high double-digits to low triple-digits and therefore still within daily fluctuations".

Germany's Justice Minister Heiko Maas said Sunday that the shocking spate of sexual assaults during New Year festivities in Cologne was organized.

"For such a horde of people to meet and commit such crimes, it has to have been planned somehow," he told Bild am Sonntag newspaper.

Thirty-one suspects, including 18 asylum seekers, are under investigation for offenses including assault and theft in Cologne on New Year's Eve with the perpetrators of sexual assaults yet to be identified, Germany's interior ministry said Friday.

"Of the 31 suspects whose names are known, 18 have asylum seeker status," said ministry spokesman Tobias Plate, citing federal police figures and indicating that their alleged offenses were "mostly theft and causing bodily harm".

Chancellor Angela Merkel said Thursday Germany must have a "fundamental" debate about how to integrate newcomers, as police identified 16 people suspected of a shocking rash of sexual assaults blamed on migrants.

As outrage grew in Germany over the assaults, which included two alleged rapes and several accounts of groping during New Year's Eve festivities in Cologne, Merkel said citizens were right to raise serious questions.

German politicians waged a heated debate Thursday over whether to make it easier to expel convicted asylum seekers, after a series of sexual assaults on New Year's Eve blamed on men of foreign origin.

Authorities have said there is no concrete proof that the perpetrators of the attacks in Cologne were asylum seekers, but critics of Chancellor Angela Merkel's open-door approach to those fleeing war have seized on the opportunity to draw a link.

German Chancellor Angela Merkel's cabinet decided Wednesday to deploy an additional 550 troops to missions against jihadist fighters in Mali and Iraq.

They will be deployed in Mali, to relieve French forces in their fight against jihadists, and in northern Iraq, to train Kurdish troops also battling IS.