Six ethnic Albanians suspected of plotting a terrorist attack inspired by extreme Islamist ideology, including two believed to have fought alongside Syrian rebels, have been arrested in Kosovo, officials said. A seventh suspect remains at large.

Authorities have grown increasingly worried about the rise of extremism in a country with a strong presence of NATO peacekeepers, including hundreds of U.S. troops. Authorities had followed the cell for three months after intercepting a call allegedly plotting an attack with another person of Kosovo descent in an unnamed European country, said the police official, who is part of a team that deals with terrorist threats.

A sniper rifle, handguns and material for making an improvised explosive device were found in suspects' houses, according to the police official.

A justice official said the suspects had been watched by video surveillance, phone tapping, and email monitoring, but gave no further details because of the ongoing investigation.

Around 150 ethnic Albanians are believed to have joined foreign fighters battling the forces of Syria's President Bashar Assad and some 12 are believed to have been killed there.

The arrest warrant seen by AP said the 7 individuals — identified as Genc Selimi, Nuredin Sylejmani, Valon Shala, Adrian Mehmeti, Musli Hyseni, Bekim Mulalli and Fidan Demolli — are suspected of "preparing a terrorist act against the safety and constitutional order" in Kosovo.

The six will be detained for a month as the prosecution gathers evidence to bring formal charges. On Tuesday local media said police and justice officials received an email threatening to launch "painful attacks" on police if they do not release the suspects.