About 15,000 Russian troops have been killed since the Ukraine invasion began, British Defense Secretary Ben Wallace told Members of Parliament on Monday, according to the British Press Association.

Russia has lost more than 60 helicopters and fighter jets, and over 2,000 of Russia's armored vehicles have been destroyed or captured, Wallace added.

President Joe Biden is nominating Bridget Brink to serve as U.S. ambassador to Ukraine, he announced Monday.

Brink is currently the U.S. ambassador to the Slovak Republic and previously served as senior adviser and deputy assistant secretary in the State Department's Bureau of European and Eurasian Affairs.

"Brink spent her twenty-five-year career in the Foreign Service focused on advancing U.S. policy in Europe and Eurasia," Biden's statement said.

U.S. Secretary of State Antony Blinken on Monday said Russia had "already failed" to achieve its stated goals in Ukraine.

"In terms of Russia's war aims, Russia has already failed," Blinken told reporters in Poland, near the Ukrainian border. "And Ukraine has already succeeded because the principal aim that President Putin brought to this, in his own words, was to fully subsume Ukraine, back into Russia to take away its sovereignty and independence. And that has not happened and clearly will not happen."

Blinken and U.S. Secretary of Defense Lloyd Austin met on Sunday with Ukrainian officials in Kyiv, the capital, becoming the highest-level U.S. officials to visit the war-torn country since Russia invaded in February.

Topics discussed during their three-hour meeting included defense assistance, further sanctions on Russia and financial support for Ukraine, according to Ukrainian President Volodymyr Zelenksyy's office.

"We appreciate the unprecedented assistance of the United States to Ukraine," Zelenskyy said, according to his office. "I would like to thank President Biden personally and on behalf of the entire Ukrainian people for his leadership in supporting Ukraine, for his personal clear position."

He added, "To thank all the American people, as well as the Congress for their bicameral and bipartisan support. We see it. We feel it."

The United States will provide Ukraine with $322 million in new aid and some diplomats will return to the war-torn country, U.S. Secretary of State Antony Blinken and U.S. Defense Secretary Lloyd Austin told Ukrainian President Volodymyr Zelenskyy in Kyiv on Sunday.

Blinken told Zelenskyy the U.S. would begin returning its diplomats to Ukraine this week, according to the senior State Dept. official. The U.S. will reopen offices in Lviv in western Ukraine, with diplomats traveling there from Poland each day, with the goal to "have our diplomats return to our embassy in Kyiv as soon as possible."

President Joe Biden will also formally nominate Bridget Brink, currently serving as U.S. ambassador to Slovakia, to serve as the U.S. ambassador to Ukraine, according to the senior State Dept. official.

Among the new assistance announced last week, the first of the new Howitzers have arrived in Ukraine, Austin told Zelenskyy, a senior defense official told ABC News.

-ABC News’ Conor Finnegan

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