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British Prime Minister Boris Johnson, in an address by video to Ukraine's parliament Tuesday, told lawmakers in the war-torn country that their military has "accomplished the greatest feat of arms of the 21st century" by keeping Russian forces outside of Kyiv. 

Johnson made the remark as the United Kingdom government announced a new $375 million military assistance package for Ukraine in its fight against Russia. The U.K. says his speech is the first from a world leader to Ukraine's parliament since the war began. 

"As you turned the Russian army back from the gates of Kyiv, you not only accomplished the greatest feat of arms of the 21st century, you achieved something deeper and perhaps equally significant: You exposed Putin’s historic folly, the gigantic error that only an autocrat can make," Johnson said.

Ukrainian President Volodymyr Zelenskyy, center, applauds as Britain's Prime Minister Boris Johnson, seen on the screen, waves during a session at Ukraine's parliament, in Kyiv, Ukraine on Tuesday, May 3. (Verkhovna Rada of Ukraine Press Office via AP)

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"Because when a leader rules by fear, rigs elections, jails critics, gags the media, and listens just to sycophants, when there is no limit on his power -- that is when he makes catastrophic mistakes," he continued. 

Johnson added that "Putin’s mistake was to invade Ukraine, and the carcasses of Russian armor littering your fields and streets are monuments not only to his folly, but to the dangers of autocracy itself." 

British Prime Minister Boris Johnson tore into Russian President Vladimir Putin during his speech to Ukraine's parliament on Tuesday, May 3. (Ukrainian Presidential Press Office via AP)

The British prime minister also said the ongoing war, which is now in its 69th day, "is Ukraine’s finest hour that will be remembered and recounted for generations to come." 

"Your children and grandchildren will say that Ukrainians taught the world that the brute force of an aggressor counts for nothing against the moral force of a people determined to be free," Johnson said. 

Zelenskyy Johnson Kyiv

Ukrainian President Volodymyr Zelenskyy, left, and Britain's Prime Minister Boris Johnson talk during their walk in downtown Kyiv, Ukraine, on Saturday, April 9. (Ukrainian Presidential Press Office via AP)

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"They will say that Ukrainians proved by their tenacity and sacrifice that tanks and guns cannot suppress a nation fighting for its independence and that is why I believe that Ukraine will win," he added. 

Following his speech, Ukrainian President Volodymyr Zelenskyy said the U.K. and Ukraine are now "brothers and sisters," according to a statement from Johnson’s office. 

Fox News' Simon Owen contributed to this report.