The tensions simmering beneath Germany's willingness to take in one million migrants blew into the open on Tuesday after reports that scores of young women in Cologne had been groped and robbed on New Year's Eve by gangs of men described by the authorities as having "a North African or Arabic" appearance.

The German authorities expressed outrage at the attacks and called them unprecedented in scale and nature, saying hundreds of young men appeared to have participated.

It was not clear that any of the men involved were recent arrivals to Germany over the last year from conflicts in Syria, Iraq, Afghanistan, North Africa and elsewhere. But the situation created a new political challenge for Chancellor Angela Merkel, whose decision to take in refugees from conflict-ridden nations opened the doors to waves of migrants last summer and fall. As the number of asylum-seekers has grown and the challenge of assimilating them has become clearer, she has come under intensifying criticism for failing to anticipate the social and economic costs of her policy.

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