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

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