ReutersUkraine says it has uncovered a Hungarian-operated spy network attempting to obtain intelligence about its defences.
The country's security service said it had detained two alleged Hungarian spies who it accused of gathering intelligence on its air defence systems.
"For the first time in Ukraine's history, the Security Service has exposed a Hungarian military intelligence network that was conducting espionage activities to the detriment of our state," the SBU said.
Hungary is a member of the European Union and Nato - both firm allies of Ukraine - but President Viktor Orban has repeatedly sided with Russia since its invasion of Ukraine. The Hungarian government has not responded to the claims.
The alleged spy network was established by Hungary's military intelligence with the aim of collecting data about the location of air defence systems in the western Zakarpattia region, which sits near Ukraine's border with Hungary, the SBU said.
The spies had also been looking into the public sentiment among local residents to predict their response in the event of a Hungarian incursion, according to the SBU.
The SBU gave no indication that the alleged plot was in aid of Russia.
The two people detained were a woman and a man who were former members of the Ukrainian military, the security service said.
The alleged spies were taken into custody at their homes, Ukrainian news agency Interfax reported, and phones and other evidence which indicated they were spies were seized during a search.
The detainees are suspected of treason and face up to life imprisonment.
