Clashes between invading Russian troops and defending Ukrainian forces in Ukraine's second-largest city Kharkiv intensified Wednesday, with shelling and strikes leaving at least 21 dead and dozens injured.

Russian forces landed in Kharkiv and triggered immediate clashes in the streets, the military said, following Moscow's relentless air assault across the ex-Soviet state.

Regional Governor Oleg Synegubov said that at least 21 people were killed and 112 wounded in shelling in the eastern Ukrainian city in the last 24 hours. The authorities have said Russian missile attacks hit the city center, including residential areas and the regional administration building.

Reports from Kharkiv said that Russian forces attacked a military medical center. The UNIAN news agency reported that they engaged Ukrainian forces, which managed to take six new Russian 5-80BWM tanks.

The airborne operation came as U.S. President Joe Biden branded Vladimir Putin a "dictator," warning the sanction campaign to cripple Russia's economy would escalate and its oligarchs were being targeted.

In Biden's first State of the Union address, he hailed the resolve of the Western alliance and voiced solidarity with Ukraine as lawmakers in the U.S. Congress gave a standing ovation to the Ukrainian people.

"A Russian dictator, invading a foreign country, has costs around the world," Biden told lawmakers in his annual State of the Union address, promising "robust action to make sure the pain of our sanctions is targeted at Russia's economy."

But as he spoke a Russian escalation was reported to be underway in Kharkiv, an apparent bid by Moscow to capture its first major Ukrainian city of the invasion.

Since Russian troops rolled into Ukraine last week to achieve Putin's mission of overthrowing the pro-Western government of President Volodymyr Zelenskyy, hundreds of civilians have been reported killed.

Russian forces have carried out a massive bombing campaign and encircled urban centers, but Ukraine insists no major city has yet been overtaken.

Russia hit a residential building in the city on Tuesday killing eight people, drawing comparisons to the massacres of civilians in Sarajevo in the 1990s and condemnation for what Zelenskyy called a "war crime."

A fire broke out on Wednesday in the barracks of a flight school in Kharkiv following an airstrike, according to Anton Gerashchenko, adviser to the Ukrainian interior minister.

"Practically there are no areas left in Kharkiv where an artillery shell has not yet hit," he was quoted as saying in a statement on Telegram.

Kharkiv, a largely Russian-speaking city near the Russian border, has a population of around 1.4 million.