According to Tanjug, two Americans and two Ukrainians allegedly attempted to use a drone on Monday to take photographs of an ornate building in downtown Belgrade that houses the headquarters of Serbian military intelligence.
Separately, in the northern city of Novi Sad, a Briton and an Afghan were detained after entering a disused military facility.
A Spanish couple were also detained in the nearby holiday resort of Fruška Gora after attempting to walk into an army depot, the report said.
Authorities did not identify the detained foreign nationals.
Tatjana Sekulić, a spokesperson for the Senior Prosecution Office, said that an initial investigation had been launched.
“This means that we are trying to ascertain what happened and how … no criminal charges have been pressed yet,” she was quoted as saying by Tanjug.
A spokesperson for the US embassy in Belgrade declined to comment. Late on Monday, the Ukrainian embassy said two Ukrainian nationals had been released without charges. Spanish and British embassy officials could not immediately be reached for comment. Afghanistan has no diplomatic representation in Serbia.
Late on Monday, Serbia’s President Aleksandar Vučić said that “the case was not particularly dangerous.”
Earlier this month, Serbian tabloids reported that an American citizen, reportedly a former member of an elite US military unit, was arrested in Belgrade while in possession of an unregistered handgun. The reports could not be verified.