An activist group says some 1,000 refugees are currently sleeping rough in the German capital because authorities in Berlin are unable to cope with the influx of people seeking asylum.

The Berlin Refugee Council says authorities have given 1,800 people vouchers for private hostels in recent months because official accommodation is full.

But the group claims that only about 600 hostel places are actually available, meaning many refugees are left out on the streets.

Berlin authorities said Wednesday that more than 3,400 refugees have arrived in the city so far in July, three times as many as during the same month last year, posing a "huge challenge."

The government estimates that the number of asylum applications in Germany will surpass 500,000 this year, compared with about 200,000 in 2014.