A group of Pakistanis and a Syrian have been attacked in Cologne amid tensions over New Year's Eve assaults in the city that have been blamed largely on foreigners, German police said.

Six Pakistani nationals were attacked on Sunday by a group of around 20 people and two of them were briefly admitted to hospital, police said.

And a Syrian man was attacked by five people on Sunday. He did not need treatment for his injuries.

Police said they received tips on Sunday afternoon about groups of people who would "seek provocation", but were still investigating whether the subsequent attacks were racially motivated and whether there was any link to the New Year assaults.

Those assaults have stoked tensions over Germany's open-door policy in the refugee crisis and prompted politicians to call for tougher laws against migrants who commit crimes.

Authorities and witnesses said the New Year's Eve attackers were among a group of about 1,000 people described as predominantly Arab or North African who gathered at Cologne's central railway station. Some broke off into small groups and groped and robbed women, police said.

Cologne police say 516 criminal complaints have now been filed with them in connection with the New Year attacks. About 40% involve allegations of sexual offences.

In a separate incident, police said two migrants - a Syrian and an Afghan - were arrested in northern Germany on Sunday on suspicion of attacking and robbing a French man who was wearing a Jewish skullcap.

Police said the 49-year-old was in a waiting room at Puttgarden ferry port on Saturday when the two men, saying "Jew" in Arabic, pushed him to the floor. They stole a bag containing cash, a bank card, a train ticket and a mobile phone.

The two men had been denied entry to Denmark the previous day because they lacked the correct papers and were waiting for a train to a refugee centre.