German police have identified three suspects in connection with attacks on women at New Year celebrations in the city of Cologne, the interior minister of the state of North Rhine-Westphalia (NRW) said.

About 90 women reported being robbed, threatened or sexually molested at New Year celebrations outside the city's cathedral by young, mostly drunk, men, police said, in events they described as "a new dimension in crime".

The police chief in Cologne has said the perpetrators appeared to be of "Arab or North African" origin, prompting right-wing groups to condemn the government for its welcoming stance towards refugees.

Government officials have cautioned against putting foreigners and refugees under "blanket suspicion" because of the assaults.

Police said the attacks occurred when about 1,000 men split into gangs as officers cleared a square to stop fireworks being thrown from the top of steps into the crowd below.

NRW Interior Minister Ralf Jaeger told a news conference no arrests had yet been made. He declined to give further details of the investigation but said he expected "a very detailed report" this week from the police.

Around 150 people gathered in front of Cologne's cathedral on Tuesday evening to protest against violence against women. One of them held a sign saying: "Ms Merkel where are you? What do you say? This scares us!"

Chancellor Angela Merkel has expressed shock at the attacks.