Belarusian President Alexander Lukashenko arrived on a state visit to China on Tuesday, according to Sputnik, a Russian state-owned news agency.

During his planned three-day visit, Lukashenko will hold a series of meetings with top Chinese officials, according to Sputnik.

Belarus has been an unofficial ally of Russia since the war began.

Lukashenko's visit comes at a time when Western officials have issued warnings about the Chinese government possibly aiding Russia in its invasion.

U.S. Secretary of State Antony Blinken and NATO Secretary General Jens Stoltenberg said they have intelligence showing China is preparing to send lethal aid to Russia.

Last week, the Chinese foreign minister visited Russian President Vladimir Putin in Russia. During remarks at that meeting, Putin said he is planning for Chinese President Xi Jinping to visit Russia this spring.

-ABC News' Natalia Shumskaia

Russian President Vladimir Putin acknowledged Russian military losses from its ongoing invasion of Ukraine during his annual meeting with the federal security service Tuesday.

"Unfortunately, comrade officers, we know that there are losses in our ranks," Putin said.

Putin called on the FSB to provide "support" to the families of soldiers who have died in the war.

"We will always remember their heroism and bravery," he added.

Putin did not say how many soldiers have died in the war and the Russian Ministry of Defense hasn't disclosed an exact number of losses since September 2022 when Defense Minister Sergey Shoigu said just shy of 6,000 troops had died.

Secretary of State Antony Blinken and other State Department officials said Russia has likely suffered 200,000 injured or dead soldiers since the start of the war.

Putin also called on the FSB to make sure its border guard "undertake special efforts to control the Russian state border with Ukraine."

A "special group" is currently deployed at the Russia-Ukraine border, made up of border agencies, the FSB air arm, the Russian Armed Forces and the Russian National Guard, according to Putin.

"Your mission is to prevent any incursions by sabotage groups and stop any attempts to smuggle weapons and ammunition into Russia," Putin told the FSB board.

-ABC News' Tanya Stukalova and Anastasia Bagaeva

Air raid sirens blared for more than five hours in several locations across Ukraine starting late Sunday and lasting until the early morning hours.

Ukrainian air forces shot down 11 out of 14 that Russia launched from the north last night, according to the Air Force of the Armed Forces of Ukraine.

Over the past 24 hours, Russians have launched five missiles and 13 air strikes, according to the General Staff of the Armed Forces of Ukraine.

The Russian troops also launched more than 50 rocket attacks primarily in Kherson and Donetsk regions, which led to "dead and wounded civilians, destroyed civilian houses and damaged civilian infrastructure," according to Ukrainian officials.

Shelling from Russian forces was recorded in more than 25 settlements in the Kharkiv and Luhansk regions, Ukrainian officials said.

-ABC News' Ellie Kaufman

U.S. Treasury Secretary Janet Yellen visited Kyiv Monday and met with Ukrainian President Volodymyr Zelenskyy to reaffirm the U.S.'s financial support to Ukraine.

Yellen announced $1.25 billion in economic and budgetary assistance, which helps the Ukrainian government continue to run, funds payroll for soldiers on the front lines and shores up critical infrastructure, according to the Treasury Department.

Yellen said the money is part of the $45 billion Congress approved for Ukraine in December.

"I bring to Kyiv a clear message from President Biden and the American people: We will stand with Ukraine for as long as it takes," Yellen said in her remarks during her sitdown with Zelenskyy.

Yellen told Zelenskyy that the U.S. will provide over $8 billion in this type of assistance "over the coming months," according to the Treasury Department. A readout of the meeting stated the U.S. will provide $9.9 billion total during the first three quarters of 2023.

-ABC News' Sarah Kolinovsky

A Moscow court has sentenced one of Russia's best-known opposition leaders, whose family live in the U.S., to 25 years in prison in what is widely seen a show trial.

Vladimir Kara-Murza is the most high-profile opponent of Russian President Vladimir Putin to be jailed since Alexey Navalny.

Kara-Murza's extraordinarily harsh sentence is one of the lengthiest any opposition figure has received under Putin and illustrates how repressive Russia has become during the war in Ukraine, reverting to something much closer to the USSR where no opposition is tolerated.

Kara-Murza was convicted of treason, as well as "discrediting Russia's armed forces," a new law that effectively criminalizes criticizing the war in Ukraine. He was also convicted of belonging to a banned organization. The charges are widely seen as politically motivated.

Kara-Murza is one of Russia's best-known pro-democracy figures and a veteran critic of Putin.

Kara-Murza, who holds both British and Russian citizenship, spent many years living in the United States and his wife and children still live in Virginia. He was close to the late U.S. Sen. John McCain, who championed human rights in the former Soviet Union.

Dozens of journalists and Western diplomats attended the court hearing on Monday, including the U.S. ambassador who read out a statement condemning the sentence.

"We support Mr. Kara-Murza and every Russian citizen to have a voice in the direction of their country. Mr. Kara-Murza and countless other Russians believe in and hope for a Russia where fundamental freedoms will be upheld. And we will continue to share those hopes and work for that outcome," Amb. Lynne Tracy said.

Kara-Murza previously has survived being poisoned not once but twice. In 2015 and then again in 2017, he suffered organ failure after being exposed to an unknown toxin. Independent researchers later linked the poisoning to the same team of FSB poisoners who targeted Navalny.

He chose to return to Russia after the war began, believing it was important to continue to campaign for freedom in his country and has been an outspoken critic of the invasion.

His trial was held entirely behind closed doors, but a letter containing his closing statement to the court has been released to reporters.

"I only blame myself for one thing," Kara-Murza said in the statement. "I failed to convince enough of my compatriots and politicians in democratic countries of the danger that the current Kremlin regime poses for Russia and for the world."

"Criminals are supposed to repent of what they have done. I, on the other hand, am in prison for my political views. I also know that the day will come when the darkness over our country will dissipate.

-ABC News' Patrick Reevell