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

Ukrainian President Volodymyr Zelenskyy said the Ukrainian counteroffensive will start before Ukraine receives F-16 fighter jets sought from Western countries.

"Frankly speaking, it would help us a lot. But we also understand that we can't drag it [the counteroffensive] out, which is why we'll start before we receive F-16 [aircrafts] or other models," Zelenskyy said at a news conference for Scandinavian media held in Kyiv on Saturday, according to a script provided by Reuters.

He added, "But to calm Russia down with the fact that we'd still need a couple of months to train on the aircrafts and only then we'd start; No, this won't happen. We'll start and go forward, while at the same time, simultaneously, I think this is very important [to receive western fighter aircrafts.]"

He said Ukraine is "capable of putting an end to this war."

Also on Saturday, Ukraine's Foreign Minister Dmytro Kuleba said Ukraine received a signal from some countries about the readiness to start training Ukrainian pilots on F-16 fighter jets.

According to Kuleba, the F-16 fighter jet is the ideal aircraft due to its technical characteristics, although Kyiv does not overlook other aircraft, either. The minister added that the decisive word on issuing F-16s will be with the United States, because these are American fighters.

-ABC News' Edward Szekeres and Max Uzol