U.S. President Joe Biden said Russia has used a hypersonic missile in Ukraine, explaining Monday night that it's "the only thing that they can get through with absolute certainty."

"It's a consequential weapon," Biden said during remarks at the Business Roundtable's CEO Quarterly Meeting in Washington, D.C. "It doesn't make that much difference except it's almost impossible to stop it. There's a reason they're using it."

Biden warned that "the more [Putin's] back is against the wall, the greater the severity of the tactics he may employ," and started to describe false flag operations by Russia, specifically its false claims that the United States and Ukraine are developing chemical or biological weapons for use against Russia.

He said those claims are a "clear sign" he's considering using those types of deadly weapons.

"He's already used chemical weapons in the past, and we should be careful of what - what's about to come," Biden said. "He knows there'll be severe consequences because of the United NATO front, but the point is, it's real."

-ABC News' Justin Gomez

Ukrainian President Volodymyr Zelenskyy acknowledged the Ukrainians defending their country in his address Monday, in which he described the Russian forces as "slaves" whose consciousness has been replaced by propaganda.

Zelenskyy called out to Ukrainians in several cities, including those trapped in the besieged city of Mariupol, telling them, "Never think even for a moment that Ukraine does not remember you."

The Ukrainian president also acknowledged the "heroes emerging among the millions" as Ukrainians continue to take up arms against the invaders.

"Once, ordinary Ukrainians, and now, fighters," Zelenskyy said. "Men and women who stand up for our state. Everywhere — in the south, in the east, in the north, in the center, in the west and abroad."

Zelenskyy urged the public to continue to stand up so the "enemy does not believe that this is a reality."

"We will make them remember that they are not welcome," he said.

-ABC News' Christine Theodorou

Russian forces have "failed to achieve a lot of their objectives on the ground" due to heavy Ukrainian resistance, Pentagon spokesman John Kirby said Monday.

"When you look at what they've managed to do in 26 days, it's not that impressive," Kirby said during a press briefing. "I think what we're seeing here is the Russians have been flummoxed, they've been frustrated, they have failed to achieve a lot of their objectives on the ground."

Kirby declined to say whether Russia’s operations had entered a "new phase" as they shift to launching artillery and missile attacks on Ukraine’s cities but said that they are stepping up their long-range bombardment "because they are essentially still stalled."

"They're lobbing an awful lot of hardware into the cities to try to force their surrender, and it’s increased over the last few days," he said, adding that the tactic has led to more civilian casualties and damage to civilian areas.

-ABC News' Luis Martinez

A Moscow court has ruled that the use of Instagram and Facebook are to be banned effective immediately under Russia's anti-extremist legislation, Russian state-run news agency TASS reported on Monday.

The court banned the operations of Meta, the company that owns Facebook and Instagram, “on the grounds of realizing extremist activity,” Judge Olga Solopova announced Monday in a Moscow court.

The ban does not apply to WhatsApp, the messenger app also owned by Meta, “as it has no option to publicly spread information,” Solopova said.

-ABC News' Christine Theodorou