WATCH: Pentagon press secretary John Kirby holds news briefing amid Russian attacks on Ukraine

Pentagon press secretary John Kirby held a news briefing amid Russia’s continued attack on Ukraine.

Watch the briefing in the player above.

Ukrainian forces battled continuing Russian efforts to occupy Mariupol and claimed to have retaken a strategic suburb of Kyiv on Tuesday, mounting a defense so dogged that it is stoking fears Russia’s Vladimir Putin will escalate the war to new heights.

“Putin’s back is against the wall,” said U.S. President Joe Biden, who is heading to Europe this week to meet with allies. “And the more his back is against the wall, the greater the severity of the tactics he may employ.”

Biden reiterated accusations that Putin is considering resorting to using chemical or biological weapons, though Pentagon spokesman John Kirby said the U.S. has seen no evidence to suggest that such an escalation is imminent.