In an interview with Bild am Sonntag, Social Democrat (SPD) leader Sigmar Gabriel advised against introducing “new and complicated procedures,” while not entirely opposing all the recent toughened rules on asylum requests.

Gabriel, also the vice chancellor, suggested that registration centres already in place to deal with migrants should offer detention and deportation facilities.

The issue has caused deep divisions in the coalition of Merkel’s centre-right Christian Democratic Union (CDU) and its left-leaning partners.

While Interior Minister Thomas de Maiziere, a CDU member, said Friday that there was wide agreement in government, many SPD politicians have voiced concerns about the plan.

“[The transit zones] amount to mass prisons. With the SPD there will never be such a thing,” SPD deputy chairman Ralf Stegner told the Passauer Neue Presse newspaper on Saturday.

Other lawmakers have suggested setting up registration centres at the borders as an alternative in order to assess a migrant’s chances of success before allowing him or her entry to the country.

Under toughened rules which came into effect on Saturday, asylum seekers will be required to spend more time in reception centres and receive allowances in kind rather than cash, while people arriving from Kosovo, Macedonia and Albania will face quicker repatriation because their origin countries have been deemed safe.

Merkel and her government have come under increasing pressure to gain control of the refugee crisis gripping Europe, as regional authorities in the country struggle to cope with the unabated numbers of migrants.