Russia is paying the price for underestimating Ukrainian resistance, the U.K. Ministry of Defense said Tuesday in an intelligence update.

"Russia's invasion plan is highly likely to have been based on the mistaken assumption that it would encounter limited resistance and would be able to encircle and bypass population centres rapidly," the ministry said Tuesday in an intelligence update. "This assumption led Russian forces to attempt to carry out the opening phase of the operation with a light, precise approach intended to achieve a rapid victory with minimal cost."

"This miscalculation led to unsustainable losses and a subsequent reduction in Russia's operational focus," the ministry added.

According to the ministry, these "demonstrable operational failings" prevented Russian President Vladimir Putin from announcing significant military success at Monday's Victory Day parade in Moscow.

Although he showed no signs of backing down, Putin did not make any declarations of war or victory in his annual speech for Victory Day, a national holiday in Russia commemorating the Soviet Union's defeat of Nazi Germany in World War II. Instead, he drew parallels between Soviet soldiers battling Nazi troops and the Russian forces fighting now in Ukraine, as he has vowed to "de-Nazify" the former Soviet republic.

"You are fighting for the motherland, for its future, so that no one forgets the lessons of the Second World War," Putin said Monday during a military parade in Moscow's Red Square.

The U.S. will temporarily suspend 232 tariffs on Ukrainian steel for one year, Secretary of Commerce Gina Raimondo announced Monday.

Ukraine's steel industry is one of the foundations of the country's economy, employing 1 in 13 Ukrainians, according to the U.S. Department of Commerce. Some of Ukraine's largest steel communities have been among those "hardest hit by Putin's barbarism," the U.S. Department of Commerce said in a press release, and the steel mill in Mariupol has become a "lasting symbol of Ukraine's determination to resist Russia's aggression."

"Steelworkers are among the world's most resilient -- whether they live in Youngstown or Mariupol," Raimondo said.

The pledge to slash tariffs "is a signal to the Ukrainian people that we are committed to helping them thrive in the face of Putin's aggression," she said, "and that their work will create a stronger Ukraine, both today and in the future."

Ukraine is currently losing about $170 million every day due to blocked ports and the country's export potential has fallen by more than half, Ukrainian Prime Minister Denys Shmysal said on Monday.

Ukraine also submitted a several-thousand-page questionnaire, the second part of the answers, that must be completed by countries aspiring to join the European Union, Ukrainian President Volodymyr Zelenskyy said Monday during his nightly address.

"It usually takes months. But we did everything in a few weeks," Zelenskyy said.

The Ukrainian president held talks with EU leaders on Monday and claimed Ukraine could be granted EU candidate status as early as June.

Russia running out of missiles

Russia has used up about half of its existing missiles during its invasion of Ukraine, Ukrainian Deputy Defense Minister Hanna Malyar said Monday. But the Russians still maintain the capacity and a certain supply of components to replenish some of their depleted arsenal, Malyar added.

The U.K. Ministry of Defense also stated in its Monday intelligence update that Russia's stockpile of precision-guided munitions "has likely been heavily depleted." Instead, the Russian military is now using "readily available but ageing munitions that are less reliable, less accurate and more easily intercepted."

Russia will likely struggle to replace the precision weaponry it has already expended, the ministry said.

British Prime Minister Boris Johnson tweeted Monday that he has "never been more certain that Ukraine will win," adding that the United Kingdom will stand "shoulder-to-shoulder with Ukraine for as long as it takes."

Fighting continued on May 9 at the Azovstal Iron and Steel Works plant while "some occupiers were walking along the streets" of the surrounding city of Mariupol parading with flags and Ribbons of Saint George, a traditional Russian military symbol, said Petro Andriushchenko, the Mariupol mayor's advisor. Russian forces on Monday tried to blow up the bridge used to evacuate people from the steel plant, trying to "cut off our defenders from the possibility to exit," Andriushchenko said.

There are still more than 100 civilians trapped inside the Azovstal plant in Mariupol, the Donetsk regional governor, Pavlo Kyrylenko, said in televised remarks on Tuesday.

-ABC News' Edward Szekeres, Yuriy Zaliznyak, Irene Hnatiuk and Uliana Lototska

President Joe Biden signed the Ukraine Democracy Defense Lend-Lease Act of 2022 into law on Monday, giving him authority to lend or lease defense equipment to Ukraine and other Eastern European nations.

It passed the Senate by unanimous consent and the House of Representatives with 417 votes.

A similar lend-lease program was enacted in 1941 to provide pivotal aid to Allied nations in the battle against Nazi Germany.

Ukrainian President Vladimir Zelenskyy tweeted that he's "grateful" to Biden for signing the law, adding, "I am convinced that we will win together again. ... Like 77 years ago."

-ABC News' Alexandra Hutzler

Pentagon spokesman John Kirby is blasting Russian President Vladimir Putin's comments at a Victory Day parade in Moscow.

Victory Day is a national holiday in Russia commemorating the Soviet Union's defeat of the Nazis in World War II. Putin said at the parade Monday about the Ukrainian invasion: "You are fighting for the motherland, for its future, so that no one forgets the lessons of the Second World War."

Kirby said that Putin "talked about this being a justified military operation. It's not."

Kirby said diplomatic options remained on the table and Ukraine posed no military threat.

"He said it was timely, that it needed to happen now. No ... he had plenty of options available to him," Kirby said.

He called Putin's comments that Russia was ridding Ukraine of Nazis "a ridiculous claim."

"What we should have heard was plans for how he's going to end the war, how he's going to move his forces out of Ukraine, and how he's going to finally respect Ukraine as a sovereign state, and a nation that bordered his, a nation that posed absolutely zero threat," Kirby said.

-ABC News' Luis Martinez

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