Berlin (AFP) - Russian Prime Minister Dmitry Medvedev has condemned German Chancellor Angela Merkel's liberal policy on migrants as "quite simply stupid", in an interview due to appear this weekend.

"It's quite simply stupid to open Europe's doors wide and invite in everyone who wants to come to your country," Medvedev said in an interview with the economic daily Handelsblatt.

"European migration policy is a total failure, all that is absolutely frightening," Medvedev added.

Merkel's immigration policy has sparked a heated public debate in Germany about the country's ability to integrate the nearly 1.1 million asylum seekers it took in last year.

In late summer 2015, she announced that Germany would stop expelling Syrian refugees and in September agreed with Austria to let in tens of thousands of migrants who were stuck in Hungary, which rejected asylum requests.

The moves were followed by a surge in asylum seekers travelling from Turkey to Greece and then up through the Balkans to Hungary, Austria, Germany and northern Europe.

Medvedev said he placed "great value on humanity" and wanted "to help refugees" who had been displaced by war.

But he added: "Among these people, there are also many, perhaps hundreds, or even thousands, of thugs who have come to Europe on a 'countdown mission'. Now they wait to be called and then they will act like robots" against Europe.

Merkel modified her position on refugees last month after a spate of assaults during New Year's Eve celebrations in Cologne blamed on asylum seekers.

Turning away from her mantra of "we will manage this" over the huge migrant influx, she has now backed changes to the law to make it easier to expel any migrants convicted of crime.