(NewsNation) — Ukraine said Tuesday that it detained a pro-Russian opposition leader and close Putin ally— oligarch Viktor Medvedchuk.

Ukraine President Volodymyr Zelenskyy posted a photo on social media of Medvedchuk sitting in handcuffs and wearing a camouflage uniform with a Ukrainian flag patch.

Medvedchuk, who leads the pro-Russian party Opposition Platform – For Life was being held under house arrest before the war began. He had disappeared shortly after Russia invaded Ukraine seven weeks ago.

Russia’s attacks on Ukraine have flattened cities and killed thousands. A strike Tuesday hit what is believed to be a culinary school near the airport in Kharkiv, Ukraine. There were no immediate reports of fatalities. Witnesses described a loud “whoosh” sound followed by an explosion.

The discovery of large numbers of civilians killed after Russian forces retreated from cities and towns around Ukraine’s capital, Kyiv, already has prompted widespread condemnation and accusations that Russia is committing war crimes in Ukraine.

Russian forces recently withdrew from Kyiv suburb Bucha after a monthlong occupation, leaving corpses of civilians with bound hands and gunshot wounds to the head in their wake.

The mayor of Bucha, a town that has witnessed some of the ghastliest scenes of Russia’s invasion, said 403 bodies have been found since Russian forces pulled out of the area.

Bucha Mayor Anatoliy Fedoruk told reporters he fears the death toll will rise as search efforts continue.

Moscow forces withdrew after they failed to take Kyiv in the face of stiff Ukrainian resistance. Russia now says it will focus on the Donbas, an industrial region in Ukraine’s east. Already, there are signs the military is gearing up for a major offensive there.

Civilians were fleeing areas of eastern Ukraine on Tuesday ahead of the anticipated offensive, while Kyiv said it was checking reports that Russian forces had used chemical weapons in the besieged port city of Mariupol.

“One of the mouthpieces of the occupiers stated that they could use chemical weapons against the defenders of Mariupol. We take this as seriously as possible,” Zelenskyy warned in an overnight address.

The United States and Britain said they were working to verify reports of chemical weapons use, as well. Russian-backed separatist forces in the east denied using such weapons, as it is banned under the 1997 Chemical Weapons Convention.

Reports say victims exposed to an unknown substance, reportedly deployed by a drone, have suffered respiratory failure.

Zelenskyy is continuing to meet with partner countries this week, asking for support, especially weapons. The Ukrainian president met with Lithuania lawmakers Tuesday, while Putin is set to meet with an ally, Belarusian President Alexander Lukashenko.

Putin insisted during a visit to Russia’s Far East that the military operation would prevail, and that foreign powers wouldn’t succeed in isolating Russia.

On Tuesday, Putin vowed that Russia’s bloody offensive in Ukraine will continue until its goals are fulfilled, insisting the campaign was going as planned despite the withdrawal.

So far, peace talks have not made progress toward ending a war that has displaced 10 million Ukrainians from their homes, driven more than 4 million from the country and left thousands dead. Russian Foreign Minister Sergey Lavrov said Monday that Moscow won’t pause its military operation in Ukraine for subsequent rounds of talks.