German authorities say they have searched around 600 brothels across the country in an effort to track down women who may have been smuggled from West Africa as part of an international human trafficking ring.

The Federal Criminal Police Office said Wednesday that Tuesday evening's raids turned up more than 100 women from West Africa and that there were indications that some were victims of human trafficking.

German investigators say the nationwide crackdown follows investigations that suggest a network of West Africans active in Germany and other European countries is involved in prostitution, human trafficking, passport forgery and other illegal activity.

About 63 per cent of the roughly 400,000 sex workers in Germany are migrants, the majority arriving from central and eastern Europe, according to 2008 figures from the European Network for HIV/STI Prevention and Health Promotion among Migrant Sex Workers, known by the acronym Tampep.

While prostitution in Germany is not illegal, migrants cannot obtain entry into Germany as a sex worker, meaning many who enter the country do so illegally.

This makes them more vulnerable to exploitation, Tampep said in their latest report on working conditions in the country.

Africa is the source of about 12 per cent of all migrant sex workers in Europe and accounts for about six per cent in Germany, according to Tampep.

Nigeria, Morocco, Cameroon, Sierra Leone and Algeria are the African countries most often cited by European officials as the countries of origin of migrant sex workers, some of whom are believed to have been brought into the countries through illegal means.

With files from the Associated Press