Some 100 people were detained in a recent special operation conducted by the Ukrainian Security Service (SBU) in Avdiyivka, Yuriy Tandit, an adviser to the head of SBU, said.

Most of these people have already been released, he said.

Tandit said on 112 Ukraine television on Monday SBU officials have conducted preventive activities to locate members of military units opposing the Ukrainian authorities.

"As a result of the inspection, some 100 people were detained, of which three people are suspected of illegal activities in subversive-investigation groups. One person is suspected of involvement in the work of foreign special services," the SBU official said.

The SBU official said such preventive activities are also planned in other cities of Ukraine located neat the frontline.

Tandit said, citing his information, that three people remain detained now.

"Most of those people have been released. Several people have been detained. Of course, criminal proceedings will be instituted on the basis of their detention," he said.

SBU conducted a special operation to avert terrorist attacks in Krasnohorivka, Donetsk region, on December 10, 2015. Eighty-five people believed to be involved in the DPR and LPR activities were detained. Most of the people detained in Krasnohorivka were later released, Tandit said.