Serbian opposition figures were recently targeted with powerful cyberespionage tools, according to one of the men and a report issued on Monday by the Canadian cyber watchdog group Citizen Lab.

The incident was the latest of several in which researchers say surveillance technology was used against students, opposition politicians and others in Serbia, which will hold a parliamentary election on Sunday.

Citizen Lab said on Monday that a joint investigation with the Share Foundation, a digital rights group, determined that Serbian academic Vladan Djokic's iPhone had been hacked using the Pegasus spyware tool made by Israeli group NSO.

At the same time, former prosecutor Rade Bajic told Reuters he received a warning from Apple on August 13 that his phone had been targeted using "mercenary spyware," although he and Citizen Lab both said they had been unable to establish with certainty whether he was hacked or how a hack might have happened.

Serbia's Interior Ministry did not return messages seeking comment. Officials there have previously dismissed allegations of cyberespionage. NSO, which was blacklisted by the US government in 2021 following years of reports of alleged human rights abuses, did not return emails. NSO, which says it only sells to governments, has previously said it would terminate commercial relationships that result in serious wrongdoing.

Apple confirmed having sent a batch of warnings to users on August 13 but did not directly address questions about Bajic's or Djokic's cases.

Both men have crossed Serbia's ruling party, which has been buffeted by two years of student-led anti-corruption protests.

Djokic, the rector of the University of Belgrade, has been under pressure from authorities following his expressions of solidarity with the students. Bajic said he was involved in prosecuting senior government officials over allegations that they improperly removed cultural protections on a historic Belgrade army compound that an investment company founded by US President Donald Trump's son-in-law, Jared Kushner, had planned to redevelop. Kushner's company pulled out of the plan late last year, saying it was doing so out of respect for the people of Serbia and the city of Belgrade.

"In my opinion, my involvement in this case was the sole reason for this unlawful spyware attack," Bajic said.

Last month, the Share Foundation said that at least 14 people, including students, activists and opposition party members, had been targeted with advanced spyware ahead of local elections in March. At least one of the devices was compromised using Pegasus, it said.