Russia's military is "now significantly weaker, both materially and conceptually," than it had been prior to its invasion of Ukraine, the U.K. Ministry of Defense said on Tuesday.

"Recovery from this will be exacerbated by sanctions," the ministry said in an intelligence update. "This will have a lasting impact on Russia's ability to deploy conventional military force."

President Joe Biden will head to Troy, Alabama, on Tuesday to visit a Lockheed Martin facility that manufactures weapons systems such as Javelin anti-tank missiles, which have been key in Ukraine’s defense against Russia.

Javelin missiles "are lightweight, portable, shoulder fired, anti-tank weapons system that can hit targets up to 2.5 miles away," White House press secretary Jen Psaki said Monday. "They're highly lethal, and we've sent over 5,500 Javelin anti-armor systems to Ukraine to support the Ukrainian people's fight for freedom.”

This facility can manufacture up to 2,100 Javelins per year, Psaki said.

She said Biden will also "deliver remarks about the security assistance we are providing, highlighting the urgency of the request to Congress to pass funding quickly to help Ukraine continue to succeed against Russian aggression and to make sure that the United States and our allies can replenish our own stocks of weapons to replace what we have sent to Ukraine."

Asked if there is any concern about depleting stockpiles if the U.S. keeps up this pace of giving Javelin missiles to Ukraine, Psaki said the Department of Defense ensures that the U.S. maintains enough to defend itself.

-ABC News' Justin Ryan Gomez

Russia's progress in Ukraine's Donbas region remains "minimal at best," with troops slowed by morale problems, supply issues and risk aversion in combat, a senior U.S. defense official said Monday.

"They are not making the progress that they had scheduled to make, that progress is uneven and incremental," the official said.

The official added, "That's not just because of Russian planning or lack of logistics -- a lot of it is because the Ukrainians have really been resisting quite well."

And Russia's gains, particularly east of Izium and in the city of Popasna in eastern Ukraine, have been fleeting, the official said.

"What we saw there in Popasna is not unlike what we've seen in other hamlets in the Donbas -- they'll move in and then declare victory, and then withdraw their troops only to let the Ukrainians take it back. So there was a lot of back and forth over the last couple of days," the official said.

Russian troops have also been leaving the Mariupol area to push north and northwest in recent days, according to the official.

"Largely the efforts around Mariupol for the Russians are now in the realm of airstrikes," the official said.

Russia is likely pushing these troops north as part of its plan to encircle and trap Ukrainian forces in the Donbas region, according to the official.

More Ukrainian troops are completing training on the U.S.-made M777 howitzer system at multiple sites outside of Ukraine, according to the official. Ukrainians have also completed training on the Phoenix Ghost drone system.

-ABC News' Matt Seyler

Russia's top general, Valery Gerasimov, spent several days in Ukraine's Donbas region last week, a senior U.S. defense official told reporters Monday.

The U.S. believes Gerasimov is now back in Russia, the official said.

The official couldn't confirm whether the general was targeted by Ukrainian forces during his visit and said the purpose of his trip is not clear to U.S. officials.

"It's certainly possible that his trip was a manner of oversight and trying to gauge for himself what was going on in the Donbas. But what he came away with, what he learned, what he transmitted to his commanders, if anything, we just don't know," the official said.

-ABC News' Matt Seyler

Mariupol has not yet fallen, despite Russia's demands that Ukrainian troops defending the besieged Ukrainian port city surrender, according to Ukrainian Prime Minister Denys Shmyhal.

"There [are] still our military forces, our soldiers, so they will fight until the end," Shmyhal told ABC News' George Stephanopoulos in an exclusive interview Sunday on "This Week."

Mariupol is a strategic city for Moscow because it would allow Russian forces in the south to connect with troops in eastern Ukraine's contested Donbas region. It would also give Moscow a key port.

Although Mariupol remains under the Ukrainian government's control, Shmyhal said the city's residents are suffering.

"They have no water, no food, no heat, no electricity," he said. "They ask all of our partners to support and help stop this humanitarian catastrophe."

-ABC News' Monica Dunn