The number of attacks and other criminal offences committed against refugee shelters in Germany has more than doubled this year compared to 2014, police said.

Germany expects a record-breaking number of 800,000 new arrivals this year. The unprecedented influx of foreigners has fuelled social tensions and there have been protests and attacks against asylum shelters in some parts of the country.

"In the last few months, the crimes have reached a new level, both in quantity and quality," Sandra Clemens, spokeswoman for the BKA Federal Criminal Police Office, said.

The number of criminal offences against asylum shelters so far this year surged to 437 from around 200 in the whole of 2014, she said, adding that damage to property, graffiti and verbal insults made up the largest part of the offences.

The total for 2015 included 59 crimes classified as violent, a new record high after last year's peak of 28, Clemens said. Of this year's figure, 26 were arson attacks.