Cologne, Germany (dpa) - A 26-year-old Algerian asylum seeker is the first person to be arrested on sexual assault charges linked to a series of New Year's Eve attacks in Cologne that have forced Germany to question the wisdom of its open-arms policy towards migrants.

The man, who also stands accused of theft, is allegedly part of a group of mostly North African men who encircled, sexually assaulted and robbed women near Cologne's main train station on December 31.

The suspect was arrested alongside another 22-year-old Algerian asylum seeker who stands accused of committing theft - but not sexual assault - on the night in question. Both were residents of a refugee hostel in Kerpen, a town just west of Cologne.

Hundreds of women have come forward since New Year's Eve saying they had been encircled, sexually assaulted and robbed by groups of mostly North African men near the city's main train station on December 31.

The recent arrests mean that police are now investigating 21 people, eight of whom are in custody. Most of these men are accused of stealing belongings such as mobile phones. The assaults were mirrored to a lesser extent in other German cities such as Hamburg and Stuttgart.

Germany allowed about 1.1 million migrants to enter the country in 2015.

The government of Chancellor Angela Merkel is pressing ahead with changes that would ease the deportation of asylum seekers convicted of certain crimes in Germany. Nonetheless, the New Year's Eve attacks have given critics of Merkel's migration policy extra ammunition in arguing that the influx has made Germany less safe.