The Czech Republic's counter-intelligence agency says there's a threat of a potential attack by Islamic radicals on Czech territory.

The agency, known as BIS, says in its annual report published Thursday that such a threat comes from militants linked to the Islamic State group and also from the Nusra Front, al-Qaida's affiliate in Syria.

The announcement comes after a wave of attacks in Western Europe.

BIS says that according to its information, seven Muslims who were in the Czech Republic for an unspecified period of time left in 2015 with the aim of joining extremist groups in Syria, especially the Islamic State movement.

The agency says their return to the country would pose a "serious security threat." It says the findings have been shared with authorities at home and abroad.