The number of dangerous Islamists in Germany has sharply risen over the past year, almost doubling to just under 500 people deemed to have the potential to carry out a terror attack.

The German Criminal Police Office has tracked 497 "threats," or individuals with extremist views who could be suspected of carrying out terrorist attacks, newspaper Die Welt am Sonntag reported.

This marks a significant increase since January 2015, when Germany registered 270 potentially violent Islamists.

The authorities said an additional 339 Islamists were being tracked as "relevant persons," or individuals who may assist and sympathise with terrorist causes.

It comes as European security authorities have warned of an increased terrorist risk in the face of the upcoming European Football Championship 2016 in France.

Patrick Calvar, the head of France's DGSI internal intelligence agency, warned last Thursday that Islamic State militants are gearing up for a campaign of bomb attacks on large crowds in France, which hosts next month's Euro 2016.

Addressing the parliament's defence committee, he spoke of a "a new form of attack ... characterised by placing explosive devices in places where there are large crowds and repeating this type of action to create a climate of maximum panic.