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

The Russian oligarch behind the Wagner private paramilitary group fighting for the Kremlin in Ukraine is threatening a mutiny if his forces are not resupplied with ammunition soon.

Yevgeny Prigozhin, owner and curator of the Wagner group, penned a letter to Russian Defense Minister Sergei Shoigue, demanding ammunition be provided to his forces on the battlefield.

In the letter, Prigozhin wrote that if supply problems are not fixed fast, he will complain to Russian President Vladimir Putin and his mercenaries would abandon their positions in Bakhmut, Ukraine, where heavy fighting has been going on for weeks, The Moscow Times reported.

“I appeal to Shoigu with a request to immediately issue ammunition. In case of refusal, I consider it necessary to convey to the Supreme Commander-in-Chief information about the existing problem in order to make a decision and about the advisability of further presence of Wagner PMC in Bakhmut in the conditions of a shortage of ammunition," Prigozhin wrote.

He added, "If the deficit is not replenished ... we will be forced to withdraw part of the units from this territory, and then everything else will crumble. Therefore, the bell is already ringing -- it is called an alarm."

Emphasizing the urgency, Prigozhin noted that Ukraine is planning to launch a counteroffensive soon.

There was no immediate public response from Shoigu or the Kremlin.

"We need to stop deceiving the population and telling that everything is fine with us," Prigozhin wrote. "I must honestly say: Russia is on the brink of disaster. If these screws are not adjusted today, the 'aircraft' will crumble in the air."