BERLIN, (AFP) - German politicians waged a heated debate Thursday over whether to make it easier to expel convicted asylum seekers, after a series of sexual assaults on New Year's Eve blamed on men of foreign origin.

Authorities have said there is no concrete proof that the perpetrators of the attacks in Cologne were asylum seekers, but critics of Chancellor Angela Merkel's open-door approach to those fleeing war have seized on the opportunity to draw a link.

Victims described their attackers as being "Arab or North African" in appearance, and 16 suspects are being investigated, but no one has yet been charged.

As outrage grew in Germany over the assaults, which included two rapes and several accounts of gropings, Interior Minister Thomas de Maiziere said foreigners who commit serious crimes "must assume they will be deported".

German law requires a conviction of at least three years in prison before an asylum seeker can be forcibly returned while his or her application for refuge is being examined.

The individual must also not face threats in his or her country of origin.

But de Maiziere said: "We will have to discuss if we should change" these rules that are in line with the UN Geneva Convention and European Human Rights Convention.

Stephan Mayer, a senior politician in the Christian Social Union -- the Bavarian sister party of Merkel's CDU -- demanded that foreigners who receive a prison term, regardless of its length, be expelled.

But the Social Democrats, junior partners in Merkel's left-right "grand coalition", argued against any changes to the law.

The SPD's deputy chief, Ralf Stegner, told Die Welt newspaper that "we need neither changes to fundamental rights for asylum nor to the Geneva Convention on refugees."

He also warned that constantly "reacting to current public moods is not a responsible manner of governance."

In a bid to calm tensions, Justice Minister Heiko Maas said that asylum seekers with convictions of one year can already be expelled -- although a grace period must be given to the individual to leave German territory on his own accord, failing which he would be forcibly returned.

Germany took in nearly 1.1 million refugees last year, with refugees from war-torn Syria making up almost 40 percent of arrivals.