Four Hayat Tahrir Al-Sham [former al-Nusra front] militants, a terrorist group that rose in Syria amid the global war on the country, have been detained in Russia, the Russian Federal Security Service (FSB) reported Friday.

One of those detained had been preparing a terrorist attack on military facilities in the Kaluga region, though his attempts were curbed by the FSB.

The FSB announced revealing and suppressing the activities of the terrorists.

"In the Kaluga region, the activities of four members of the clandestine cell of the international terrorist organization Hayat Tahrir Al-Sham were revealed and suppressed, including one who planned, by order of foreign emissaries, to commit a terrorist act at military facilities in the region," a statement said.

The FSB found components of an improvised explosive device, instructions for making a bomb, and correspondence with terrorists located in the Middle East, the statement added.

It is noteworthy that it was found that the terrorists coming from Syria were going into Ukraine to fight against Russia alongside Kiev.

Close to 450 extremist Arab and foreign nationals have arrived in Ukraine from Idlib to fight against Russia's forces, less than only three days after they left Syria, passing through Turkey.

Relatives of extremists that have arrived in Ukraine told Sputnik that senior fighters from terrorist group Hayat Tahrir Al-Sham (the rebranded version of Jabhat Al-Nusra, i.e Al-Qaeda) have held a number of meetings with senior leaders in the Turkistan Islamic Party group and Ansar Al-Tawhid and Hurras Al-Din groups, and agreed on allowing a number of all their fighters to enter Ukraine through Turkish soil.