President Joe Biden forcefully condemned the Kremlin's brutal invasion of Ukraine and urged the world to stand firm behind efforts to repel the aggression, telling the United Nations that Russian leader Vladimir Putin was "reckless" in issuing a veiled threat about using his nuclear arsenal.  .

Russia's attacks on schools, hospitals and rail stations "should make your blood run cold," Biden said Wednesday at the United Nations hours after Putin tried to boost Russia's flagging fortunes in Ukraine by authorizing a partial mobilization of reservists.

In a televised address announcing Russia's first call-up of reservists since World War II, Putin accused the West of nuclear blackmail and said his government will use all its means to protect the country.

"Those who are attempting to blackmail us with nuclear weapons must be aware that the prevailing winds may also turn towards their side," Putin said.

►Russian proxies in Donetsk province, part of the Donbas region that Russia seeks to claim, have conscripted up to 500 steel plant workers in Donetsk, essentially shutting down two steel plants, Ukraine officials said.

Two Americans fighting for Ukraine have been freed in a prisoner exchange after being captured in June in the Kharkiv region, sources close to them said Wednesday.

"We deeply appreciate everyone's prayers and especially the close communication and support of our elected officials, Ukrainian Ambassador (Oksana) Markarova, and our members of the US embassies in Ukraine and Saudi Arabia and the US Department of State,” she wrote.

Putin's plan, which starts by calling up reservists who previously served in the army, was unveiled hours after Russian-controlled regions in eastern and southern Ukraine announced plans to hold votes on becoming permanent parts of Russia.

Initially those brought back into the military will include specialists and others with "relevant experience," Putin said.

Ukraine has blamed Russia for the bombings, which international regulators warn could trigger a nuclear disaster.

national security adviser Jake Sullivan said Russia would manipulate "sham" results to annex the land

"Let me be clear, if this does transpire, the United States will never recognize Russia’s claims to any purportedly annexed parts of Ukraine," Sullivan said

Any prospects for talks with Ukraine after the Donbas region votes to join Russia will be impossible, the Russian lower parliament's Foreign Affairs Committee chairman said