At least 15 suspected members of an Islamist group have been arrested during police raids in six European countries, the Italian authorities report. The Islamists are said to have been preparing terrorist attacks in northern Europe and the Middle East.

Police issued 17 arrest warrants and 15 suspected Islamists were immediately detained and face terrorism charges. Almost all of them are Kurds, except one who is from Kosovo. The location of one suspect is unknown, while another is believed to be in Iraq.

The arrests took place in Italy, Britain, Norway, Finland, Germany and Switzerland on Thursday.

“This was an incredibly difficult and complicated investigation that has been going on for five years,” said prosecutor Franco Roberti, the head of Italy's anti-mafia and anti-terrorism unit, as cited by Reuters.

The arrested are said to belong to the Rawti Shax (“The New Course”) group, a network organization whose cells communicate via the internet. The group, also known as Didi Nwe (Towards the Mountain), planned to carry out attacks on Norwegian and British diplomats in the Middle East as well as politicians in Norway.

The group also sought to topple the Iraqi Kurdistan government to replace it with the rule of Islamic sharia law. It provided logistical and financial support to send jihadists to Iraq and Syria, hoping that they will be trained there for a future conflict in Iraqi Kurdistan.

"During the course of our investigation we saw some fighters leave for Syria and die in the conflict," Rome prosecutor Giancarlo Capaldo said, according to Reuters.

The members of the group also wanted to free their leader, Mullah Krekar, who is being held in a Norwegian jail. He was sentenced for issuing death threats, encouraging others to commit murders, and praising the Charlie Hebdo massacre.

General Giuseppe Governale, from Italy’s police described the raid as “the most important police operation in Europe in 20 years,” the Telegraph reports.