Ukraine has been monitoring multiple flights between Russian and Chinese cities during which the aircrafts' transponders are temporarily switched off, according to a senior Ukrainian official, who called it a cause for concern.

The official said the belief is that China could be supplying Russia with electrical components that Moscow needs for military equipment, thus diminishing the impact of Western sanctions.

The senior official, who spoke exclusively to ABC News on the condition of anonymity, added that Ukraine currently has "no proof" that China is supplying weaponry or ammunition to Ukraine.

The official also dismissed the notion of a Chinese-brokered peace plan in the near future and said Ukraine is focused on retaking more land from Russia and is preparing for a fresh offensive "in the spring or early summer."

-ABC News' Tom Burridge

Russian President Vladimir Putin visited Mariupol to inspect a number of locations in the city and talk to local residents, the Kremlin press service said on Sunday.

Putin travelled by helicopter to the Ukrainian city, which has been occupied since last year by Russians. He drove a vehicle along the city's streets, making stops at several locations.

The visit was Putin's first to newly annexed territories.

Deputy Prime Minister Marat Khusnullin reported to Putin about construction and restoration work. In the Nevsky area, a newly built residential area, Putin talked with residents. He went inside a home at the invitation of one of the families.

Putin also inspected the coastline of the city in the area of a yacht club, a theater building that was heavily bombed with civilians sheltering inside and other memorable places of the city.

-ABC News' Tanya Stukalova

Russian President Vladimir Putin traveled to Crimea to mark the ninth anniversary of the Black Sea peninsula's annexation from Ukraine on Saturday, one day after the International Criminal Court issued an arrest warrant for the Russian leader accusing him of war crimes.

Putin visited an art school and a children's center.

Russia annexed Crimea from Ukraine in 2014, a move that most of the world denounced as illegal. Ukrainian President Volodymyr Zelenskyy has demanded that Russia withdraw from the peninsula as well as the areas it has occupied since last year.

Putin has shown no intention of relinquishing the Kremlin's gains. Instead, he stressed Friday the importance of holding Crimea. "Obviously, security issues take top priority for Crimea and Sevastopol now," he said, referring to Crimea's largest city. "We will do everything needed to fend off any threats."

-ABC News' Edward Szekeres

President Joe Biden called the arrest warrant issued for Russian President Vladimir Putin Friday by the International Criminal Court "justified," though acknowledged it might not have strong teeth.

"Well, I think it's justified," Biden told reporters Friday evening. "But the question -- it's not recognized internationally, by us either. But I think it makes a very strong point."

In a earlier statement on the warrant, the White House said it supports "accountability for perpetrators of war crimes."

"There is no doubt that Russia is committing war crimes and atrocities in Ukraine, and we have been clear that those responsible must be held accountable," National Security Council spokesperson Adrienne Watson said in the statement.

-ABC News' Cheyenne Haslett and Davone Morales

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