The German chancellor said "the fundamentals of cultural co-existence" must be discussed, as more cases of assault by alleged migrants are reported in Cologne and other European cities.

German chancellor Angela Merkel said the mass sexual harassment of women in Cologne on New Year's eve raised "some serious questions" about "cultural co-existence in Germany", while it appears that women were similarly assaulted in other European cities.

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Finnish police said that "widespread sexual harassment" was reported in Helsinki during New Year's celebrations on the city's central square. "This is a completely new phenomenon," the Finnish capital's deputy police chief told AFP news agency.

"The suspects were asylum seekers. The three were caught and taken into custody on the spot," he said, adding that two cases were filed.

In Zurich, six women reported being assaulted by "several dark-skinned men". "It's a little bit similar" to what happened in Cologne, the Swiss city's police spokesman told AFP, although the scale of the alleged attacks was "difficult to compare".

In Cologne, 120 complaints have been filed for robbery, sexual abuse and rape after dozens of reportedly mainly North African and Arab-looking men attacked women at the city's main train station. Police said it was investigating "16 young men, mostly of North African origin".

Similar incidents were also reported in Hamburg, where 70 complaints of sexual assault have been filed, and Dusseldorf.

"What happened at New Year's is completely unacceptable," Merkel said Thursday (7 January) at a press conference in Berlin.

"The feeling women had in this case of being completely defenceless and at mercy is for me personally intolerable and so it is important that everything that happened must come out into the open," she said.

In her New Year's address broadcast a few hours before the incidents, Merkel had asked Germans to consider the arrival of migrants and refugees as "an opportunity for tomorrow" and warned them against division and hatred.

She had also said that Germany's "values, traditions, sense of justice, language, laws [were] the basic requirement for a good common life imbued with mutual respect".

Faced with incidents that could trigger a backlash against migrants and her policy of welcoming them, Merkel said Thursday that they raised "some serious questions that go beyond Cologne".

She said Germany "must decisively confront" the behaviour of the assaulters, adding that she did "not believe that these are only individual cases".

"We must consistently discuss the fundamentals of cultural co-existence in Germany," Merkel said, admitting that "people [were] rightly expecting action".

The chancellor also warned that Germany needed "to re-examine if everything necessary has been done with regards to expulsions to send a clear signal to those who do not respect our law".

Her vice-chancellor Sigmar Gabriel, from her coalition partner, the Social Democratic Party, proposed that convicted foreigners serve their sentence in their country of origin.

"We must examine all the possibilities of international law to send back criminal asylum seekers to their country," Gabriel told Bild newspaper Friday.

"Why should German taxpayers pay for the prison term of foreign criminals," he said, adding that countries of origin should see aid being cut if they refuse to take their convicted nationals back.

New Year's harassment cases have also spurred debate in Belgium, where the asylum and migration minister, Flemish nationalist Theo Francken, said he planned to introduce classes to teach migrants respect towards women and how to behave with them.

"We'll copy the Norwegian model and introduce these classes in the coming weeks in all our reception centres," Francken told Belgian media.

"This decision is based on an incident that occurred in Germany for which we have no convincing element …. This is racism under thin disguise," said the minister for women's rights in Wallonia, Socialist Isabelle Simonis.