Latvia has detained two of its citizens since June on suspicion of spying for Russia's military intelligence agency and using a private aircraft to surveil an army base hosting NATO forces, authorities said Thursday.

Latvia's State Security Service (VDD) said the two Latvian citizens are suspected of collecting information on the Ādaži base — one of Latvia's most important military stations and home to NATO's brigade in the country — as well as military radar systems and the movement of planes.

According to the VDD, the pair gathered information for Russia's GRU military intelligence service. One of the suspects even used a private aircraft to conduct reconnaissance and take aerial photographs of the base.

Authorities allege that the pair carried out activities that could support Russian intelligence and sabotage groups operating in Latvia.

The case comes amid heightened concern across NATO's eastern flank over Russian sabotage and other hybrid operations targeting the government's support for Ukraine. Criminal proceedings were opened on June 27 and the investigation is ongoing.