Biden says Russia has used hypersonic missile in Ukraine, Putin's 'back against the wall'

March 21 (UPI) -- President Joe Biden says Russia has used hypersonic missiles in its war against Ukraine after Russian forces were slowed by stiff Ukrainian resistance.

The accusation came after Pentagon officials said that it was unclear whether Moscow had used hypersonic weaponry -- which travel at speeds of at least Mach 5 and are five times faster than the speed of sound and considerably faster than traditional missiles.

Biden made the remarks at the Business Routable's CEO Quarterly Meeting on Monday, saying that Moscow resorted to the hypersonic missile "because it's the only thing that they can get through with absolute certainty."

"As you all know, it's a consequential weapon," he said.

Biden added that the stalled Russian advances in parts of Ukraine have pushed Russian President Vladimir Putin to use more advanced weapons.

"Putin's back is against the wall," he said. "He wasn't anticipating the extent or the strength of our unity. And the more his back is against the wall, the greater the severity of the tactics he may employ."

Russia began the invasion on Feb. 24, but it's been partly stalled in some locations like Kyiv by a strong Ukrainian resistance that has support of much of the democratic world.

The Russian defense ministry said in a video message published to Twitter that it used hypersonic missiles on Friday to strike an ammunition storage facility in western Ukraine.

British defense officials said Monday that it's likely Moscow used Kinzhal, or dagger, missiles, which Russia has said has a top speed of Mach 10. They added that Russia's announcement of having used Kinzhal missiles is "likely intended to detract from a lack of progress in Russia's ground campaign," according to an intelligence update.

"Deployment of Kinzhal is highly unlikely to materially affect the outcome of Russia's campaign in Ukraine."

The Congressional Research Service said in a paper published last week that Russia has conducted research on hypersonic weapons since the 1980s, but has accelerated efforts due to the deployment of U.S. missile defense in Europe. In recent years, Moscow's military has announced several successful hypersonic tests.

Biden's remarks came after two Pentagon officials said they were unable to verify that the missile was used in Ukraine.

"We've seen the Russian claim that they used a hypersonic missile. We're not in a position to refute that claim, but we're also not able to independently verify it," Pentagon spokesman John Kirby told reporters in a press briefing Monday.

An unnamed senior defense official said the same thing during a separate briefing, but added that it's "certainly possible" a hypersonic missile was used.