(AFP) - Marianna Budanova, who is an advisor to Kyiv mayor Vitali Klitschko, was hospitalised after a prolonged deterioration in her health, the Babel news outlet reported earlier.
Ukrainian investigators' "main hypothesis" is that Russia was involved in the poisoning attempt, Ukrainian military intelligence spokesman Andriy Yusov later told AFP.
"The target was the wife," he added, because "it is simply impossible to reach the commander directly."
There were no immediate comments from Russian authorities.
Yusov said mercury and arsenic, highly toxic substances, were used in the attack.
Citing unnamed intelligence sources, Babel said Kyiv had opened an investigation into what it described as "attempted murder".
A source speaking on condition of anonymity told AFP Babel's report was accurate, adding that Budanova had been poisoned and was receiving treatment, but declined to give further details.
Babel reported that the substances found in her body "are not used in normal life or military affairs".
"Their presence may indicate a deliberate poisoning attempt," it said. It also said "several" other GUR employees were being treated for suspected poisoning.
Yusov confirmed to AFP that traces of heavy metals had been found in other employees, but declined to say how many.
'Over time'
Budanova was reportedly hospitalised after her condition deteriorated and has already undergone some treatment.
It is not clear when she fell ill, but Yusov told AFP that the assassination attempt "could have been stretched over time."
"I can't talk about the timing now," he said.
Law enforcement officials believe the poison was administered through her food, the Ukrainska Pravda reported.
Russia has been accused of poisoning attacks before, although the Kremlin has firmly denied the charges.
In 2018, a former Russian military intelligence officer convicted by Moscow of high treason was poisoned along with his daughter in the United Kingdom.
Assassinations are not unheard of in the Ukraine war.
Several pro-Russian officials and supporters of Moscow's invasion of Ukraine have been assassinated since Russia launched its full-scale assault on Ukraine last February.
Officials have said they have foiled "more than 10" assassination attempts against Budanov, a highly respected figure in Ukraine.
Budanov's influential military intelligence unit is considered responsible for several sabotage attacks against Russia that have taken place behind the frontlines.
Moscow has accused the GUR of being behind the October 2022 explosions on the Kerch bridge, which links Russia to the annexed Crimean peninsula.
Budanov said in August his wife had been living with him "in his office" and had not left his side since the start of the invasion for security purposes.