The number of radical Islamists in Berlin has nearly doubled in five years, with many of them categorised as "potentially violent" including at least 50 who have traveled to and from the territories occupied by the Islamic State (ISIS) to fight for the group, according to a top German official.

From 350 identified five years ago, the number is now pegged at 680 hardcore Islamists, with nearly half of them "geared toward violence,'' said Isabelle Kalbitzer, spokesman for the Berlin office of the Federal Office for the Protection of the Constitution, Germany's equivalent of the FBI.

"We consider the ideology of the 680 to not be compatible with our freedom and that can be dangerous," Kalbitzer told Fox News, adding that the 50 estimated to have returned from ISIS pose the most immediate threat.

The entrenched radicals reportedly found a new audience in the growing pool of Muslim migrants and refugees who have flocked to Berlin from the Middle East and northern Africa.

According to Kalbitzer, the violent Islamists grew in number by generally recruiting from among Muslim migrants and refugees. The recruitment is often being done in Berlin's mosques and even in front of the city's main refugee registration centre.

During Friday worship in the city's mosques, the official said firebrand imams can be heard delivering "poisonous'' messages to convince the refugees to join them.

In addition, Kalbitzer said the Muslim Brotherhood, which has been designated as a terrorist organisation by the U.K. and Egypt, has a recruitment presence at the Lageso refugee centre in the heart of Berlin.

"There are meeting places where the Salafists are recruiting," she added said. "It continues to happen in different circles."

The Berlin Integration Minister Dilek Kolat said Berlin is a stronghold of the Salafism, "a virulent and violent strain of Sunni Islam that gave rise to groups such as Al Qaeda and ISIS,'' the newspaper Berliner Morgenpost reported.

Estimates from the federal law enforcement say there are 7,000 active Salafists in the country, "animated by the goal of a return to 7th-century Islamic society."

"They openly advocate stripping women of their rights and an austere way of life governed by a harsh form of Islamic Sharia law. They can be seen in Berlin handing out copies of the Quran and propaganda materials,'' she said.

Nearly 50,000 refugees have landed in Berlin, most from Muslim-majority countries such as Syria, Afghanistan, Egypt and Iraq, said the report.