BERLIN (AP) — The number of Germans who reject the legitimacy of the government has increased sharply over the past 18 months, Germany's domestic intelligence agency said Tuesday.

The Federal Office for the Protection of the Constitution, or BfV, said in its annual report on extremism in Germany that the Reichsbuerger movement increased to about 16,500 people last year from about 10,000 in 2016. By July of this year officials estimated the movement had grown further to about 18,000 people, the agency's head Hans-Georg Maassen told reporters.

The Reichsbuerger, or Reich Citizens, movement doesn't accept the boundaries and laws of post-war Germany, and its adherents often clash with officials and police. Many identify with the German Reich of 1871-1918 or assert a form of self-rule similar to the sovereign citizens movement in the United States.

Most of those identified as Reichsbuerger are male, over 40 and strongly attracted to firearms, the agency said, but only about 5 percent are considered far-right extremists.

Still, the head of Germany's Central Council of Jews called the movement's growth "particularly worrying" because of its anti-Semitic tendencies.

"This shouldn't just worry the Jewish community, but the entire country," said Josef Schuster.

He also noted the links between the far-right Identitarian Movement and the anti-migrant Alternative for Germany party, which came third in last year's national election. "They could become a bridge for extreme right ideas to enter our parliaments," said Schuster.

Concerns have been raised, too, about the presence of Alternative for Germany supporters or members within the country's intelligence services.

"I have no information about AfD members at the Federal Office for the Protection of the Constitution , or of an infiltration by AfD," said Interior Minister Horst Seehofer, referring to the party by its acronym.

The agency's report also recorded an increase in the number of people in Germany who follow the fundamentalist Islamic Salafist ideology to 10,800, from 9,700 the previous year.