German Chancellor Angela Merkel has voiced her outrage over a string of "disgusting" sexual assaults that male gangs committed against women in Cologne on New Year's Eve.

As many as 1,000 young men, many of whom were said to be drunk and of Arab or North African appearance, harassed women in the area around Cologne's central station on 31 December, prompting 90 complaints to the police.

The attacks shocked Germany, and prompted Merkel to tell Cologne Mayor Henriette Reker about her "outrage" over the "disgusting attacks and sexual assaults".

Merkel added that the numerous perpetrators of the attacks, who stole from and harassed large numbers of women, must be found "as quickly and comprehensively as possible".

She added that Germany must punish them, "regardless of their origin or background."

At least one woman was raped in the city, and large numbers were groped during what the Mayor called "monstrous" attacks, including one volunteer policewoman.

Women protest against sexism outside Cologne Cathedral on 5 January after the assaults (Oliver Berg/EPA)

There was no official confirmation that refugees and asylum seekers were involved, but one police officer told local news site Express that he had detained eight men, all of whom were asylum seekers.

Express also reported that five men, from the ages of 18 to 24, had been arrested, although their origins and alleged crimes are unknown.

Despite the debate in Germany over the hundreds of thousands of refugees who arrived last year, Mayor Reker said it was "completely improper" to link the group to the newly-arrived refugees.

Cologne is currently gearing up for large carnivals in the city centre in February, and Mayor Reker promised an increased police presence, warnings to young women about potential dangers and "better explanation" to asylum seekers about the meaning of the annual carnivals, which like New Year's Eve, are typically drunken events.

NYE celebrations in Cologne