Far-right extremists in Germany are joining forces with like-minded groups across Europe and even the United States as they prepare to carry out more attacks, the country's intelligence chief has warned.

Hans-Georg Maassen, the head of the Verfassungsschutz, said the agency had been forced to step up its efforts to foil plots by neo-Nazi and fascist groups following a surge in extremist violence in 2015.

"This is not just purely a German phenomenon," he told Reuters, "the Right-extremist scene is networking on a European level, and in some cases, with connections in the United States."

"We have seen in a series of cases that there are numerous people in the far-Right extremist scene who are ready to do anything and who have joined forces to create Right-wing terrorist cells."

He added: "We are trying to investigate these cells, if they exist, and to prevent any attacks."

Mr Maassen's remarks came after the Verfassungsschutz's annual report revealed that cases of far-Right violent acts in Germany had increased by 42 per cent to 1,408 in 2015.