Ukraine's security service has said it launched a criminal probe under a law on information gathering after a "technical device" was found in an office that could have been used in the future by Commander in Chief Valery Zaluzhnyi.
The security service said on the Telegram messaging app that an investigation had been opened under an article in Ukraine's criminal code on "unlawful acquisition, sale, or use of special technical means for obtaining information".
It added that the device, initially characterised as a bug by local media, was considered under preliminary information to be "in a non-operational state", and no means of information storage or remote transmission of audio recordings were found.
"We emphasise that the device was found not directly in Valery Zaluzhnyi's office but in one of the premises that could have been used by him in the future for work," the service said.
"The Security Service of Ukraine is taking all legally prescribed measures to investigate this offence," it added.
Mr Zaluzhnyi has been mentioned among possible targets for a reshuffle in top military ranks under a cooling in relations with President Volodymyr Zelenskiy, who has disagreed with his view of the conduct of the 21-month-old war against Russia.
In an essay published last month in The Economist, Mr Zaluzhnyi wrote that the war was entering a stage of attrition that could benefit Moscow and allow it to rebuild its military power.
Volodymyr Zelensky has disagreed with any notion the war was subject to a stalemate.
Putin says remark about plan to attack NATO is nonsense
Meanwhile, Russian President Vladimir Putin has said US President Joe Biden's claim that Russia would attack a NATO country if he won in Ukraine was complete nonsense, adding that Russia had no interest in fighting with the NATO military alliance.
In a plea to Republicans not to block further military aid earlier this month, Mr Biden warned that if Mr Putin was victorious over Ukraine, then the Russian leader would not stop and would attack a NATO country.
Mr Biden offered no clear evidence for his remarks.
"It is complete nonsense - and I think President Biden understands that," Mr Putin said in an interview published today by Rossiya state television, saying it was an attempt by Mr Biden to justify his "erroneous policy" on Russia.
"Russia has no reason, no interest - no geopolitical interest, neither economic, political nor military - to fight with NATO countries," he said.
The US-led NATO alliance was founded in 1949 to provide Western security against the Soviet Union.
After the 1991 collapse of the Soviet Union, it was enlarged to include some former Soviet and Warsaw Pact countries.
Mr Putin has repeatedly cast the post-Cold War expansion of NATO as evidence of the West's arrogant way of dealing with Russia's security concerns.
The Russian president also warned of "problems" with neighbouring Finland after it joined NATO earlier this year, saying Moscow will create a new military district in north-west Russia in response.
Finland, which shares a 1,340km border with Russia, joined NATO in April this year in the midst of Moscow's Ukraine offensive.
"They (the West) dragged Finland into NATO. Did we have any disputes with them? All disputes, including territorial ones in the mid-20th century, have long been solved," Mr Putin told a state TV reporter in the same interview.
"There were no problems there, now there will be, because we will create the Leningrad military district and concentrate a certain amount of military units there," he added.
Later, in his first campaign speech before running for president again in March, Mr Putin promised to make Russia a "sovereign, self-sufficient" power in the face of the West.
"We must remember and never forget and tell our children: Russia will be either a sovereign, self-sufficient state, or it will not be there at all," Mr Putin said during a congress of the ruling United Russia party.
Earlier, debris from a downed drone killed a civilian in the Odesa region, the governor of the southern Ukrainian region said today after Ukraine's air force said it had destroyed 20 drones that Russia launched overnight.
Air defence systems destroyed nine Iran-made attack drones over Odesa, Governor Oleh Kiper said on the Telegram messaging app, calling it the third Russian air assault on the region in the past week.
"However, one of the downed drones fell into a residential area in Odesa district and exploded," Mr Kiper said, adding that several houses were damaged and one person was found dead in one of them afterwards.
The air force said its defence systems destroyed a cruise missile in addition to the drones. It said, without providing details, that Russia also launched an Iskander ballistic missile that "did not reach" its target.
The Russian air weapons were destroyed over Odesa, Kherson, Zaporizhzhia and Khmelnitskyi regions, the air force said.
Reuters could not independently verify the report. It was not clear how many drones Russia launched in total.
