A string of sex assaults and robberies during New Year's celebrations in Germany has fuelled debate about the country's ability to integrate large numbers of refugees after police said that men who targeted dozens of women in the western city of Cologne appeared to be of "Arab or North African origin."

Political leaders including Chancellor Angela Merkel condemned the attacks, though many also warned against hasty conclusions about the perpetrators. But to some Germans already uneasy about the one million asylum-seekers their country took in last year the incident seemed to confirm simmering fears.

"Is this the 'cosmopolitan and colorful' Germany that Merkel wished for?" asked Frauke Petry, leader of the nationalist party Alternative for Germany.

Petry's party, known by its German acronym AfD, has called for a clampdown on the number of asylum-seekers allowed into the country, a sentiment shared among a growing number of supporters in Merkel's own center-right bloc.

"It's unacceptable that women are sexually molested and robbed by young migrants on the streets and public squares of German cities at night," said Andreas Scheuer, general secretary of the Christian Social Union, the Bavarian wing of Merkel's party.

"Whoever won't accept our rules for living together, including respect for women, can have no place in our society here in Germany," said Scheuer. His party has called for a cap of 200,000 asylum-seekers in Germany a year, a demand its lawmakers are likely to repeat at a meeting with Merkel on Wednesday.

Others in Germany cautioned against tying the refugee question to the issue of street crime when the full facts of the incident aren't known yet.

"It's completely improper ... to link a group that appeared to come from North Africa with the refugees," Cologne's mayor Henriette Reker told reporters after a crisis meeting with police Tuesday. Syria, Albania and Kosovo were the top three countries of origin for asylum-seekers in Germany last year.

Cem Ozdemir, a Green Party lawmaker of Turkish origin, described the attacks as "horrible and deeply misogynist."

"Women must feel safe everywhere, no matter where or when," he said.

Cologne's police chief Wolfgang Albers said no arrests have been made yet. "We don't currently have any suspects, so we don't know who the perpetrators were. All we know is that the police at the scene perceived that it was mostly young men aged 18 to 35 from the Arab or North African region."

Albers urged witnesses to come forward, especially if they recorded videos of the attacks. At least 90 criminal complaints have been filed, including one allegation of rape, police said.

Police said the attackers had gathered in large numbers near the city's main train station, drinking alcohol, releasing fireworks and mingling with other revelers.

Separately, police in the northern city of Hamburg appealed for witnesses who observed similar sexual assaults and thefts in the St. Pauli district on New Year's night.

German Justice Minister Heiko Maas said the attacks shouldn't be used to bolster an anti-refugee agenda. "In criminal law what's important is proving a crime, and everyone is equal before the law," Maas said. "It doesn't matter where someone comes from, it matters what they did and that we can prove it."

On Tuesday evening some 300 protesters gathered near the site of the assaults in front of Cologne Cathedral, next to the train station. One woman held a hand-made placard that read: "Mrs Merkel, where are you? What do you say? This is scary."

Merkel's office said the chancellor had called mayor Reker earlier in the day and "expressed her outrage about these despicable assaults and sexual attacks, that demand a hard response by the forces of law."

She also called for everything to be done "to find the perpetrators as quickly and comprehensively as possible and to punish them without regard to their origin or background," her office said.

German authorities have regularly dismissed the idea that the influx of refugees is leading to a disproportionate rise in crime. At the same time, security officials have warned that violence against asylum-seekers has increased steeply over the past year.

Federal police said Tuesday that they had detained a 26-year-old man who was traveling to Cologne with a foot-long meat cleaver in his pocket.

Police said the unidentified man told officers: "I'm on the way to Cologne to get an idea of the sexual assaults. I need the cleaver for that." The man was taken to a psychiatric hospital.

Cologne, which was founded by the Romans almost 2,000 years ago and is one of the most ethnically diverse cities in Germany, plans to step up measures to prevent similar attacks during the annual Carnival festivities next month, when alcohol-soaked street parties are the norm.

"We will also have to explain our Carnival better to people from other cultures, so that there's no confusion about the cheerful behavior in Cologne that has nothing to do with candor, especially candid sexuality," Reker said.