German Chancellor Angela Merkel faced growing pressure to harden her line on refugees on Monday as the first extensive police report on New Year's Eve violence in Cologne documented rampant sexual assaults on women by gangs of young migrant men.

Cologne police said at least 11 foreigners, including Pakistanis, Guineans and Syrians, had been injured on Sunday evening in attacks by hooligans bent on revenge for the assaults in the city.

Interior Minister Thomas de Maiziere condemned those attacks and warned against a broader backlash against refugees following the events in Cologne, which have deepened skepticism toward Merkel's policy of welcoming migrants.