Russia-Ukraine updates: Putin says 'certain positive movements' in negotiations

Ukraine’s military on Sunday said it was "fighting fierce battles to maintain certain borders," including defending Chernihiv and Mariupol, cities targeted by Russian shelling.

"The enemy, suffering losses, makes constant attempts to avoid direct fire contact with our troops," military officials said, according to an official translation posted on Facebook.

An operation is underway to defend Chernihiv, a city about 100 miles north of Kyiv, the capital, the update said. In the east, the Ukrainian military’s "main efforts are focused on defending the city of Mariupol and inflicting fire damage on the overwhelming forces of the enemy."

Ukraine’s military claimed on Sunday to have destroyed 88 Russian aircraft and helicopters.

"Some enemy pilots who catapulted and survived were found by Ukrainian soldiers," the update said. "They have received medical treatment and are already testifying about the Putin regime's crimes against humanity."

Russia has responded to the “scale and strength” of Ukraine’s resistance by targeting populated areas, including Kharkiv, Chernihiv and Mariupol, the U.K. Ministry of Defence said on Sunday.

"This is likely to represent an effort to break Ukrainian morale," the ministry said. "Russia has previously used similar tactics in Chechnya in 1999 and Syria in 2016, employing both air and ground-based munitions."

The Defence Ministry said Russian fuel trucks and supply lines have been targeted, "slowing the rate of advance of their ground forces."

President Joe Biden called Ukrainian President Volodymyr Zelenskyy Saturday evening, according to the White House, adding that they spoke for 32 minutes.

They spoke from 6:18 p.m. ET to 6:50 p.m. ET, the White House said.

The White House released a readout of Biden's call with Zelensky, saying the leaders discussed recent talks between Ukraine and Russia and Biden "reiterated his concern" about Russia's attack on a nuclear power plant. Biden also noted assistance being sent to Ukraine and said that the administration is "working closely with Congress to secure additional funding."

Biden also "welcomed the decision this evening by Visa and Mastercard to suspend service in Russia."

Earlier in the day, Zelenskyy spoke virtually to about 300 members of Congress and their staff. He pleaded passionately for more assistance from America and the West, specifically asking for war planes, according to those in the meeting. There was no mention of that in the readout from the presidents' conversation.

-ABC News' Justin Gomez

Visa and Mastercard have suspended their operations in Russia in response to its invasion of Ukraine, the companies said Saturday.

The announcements came hours after Ukrainian President Volodymyr Zelenskyy called for a suspension of Visa and Mastercard transactions in Russia during a Zoom call with U.S. lawmakers Saturday, according to a source on the call.

Visa said in a statement it is working to end all transactions with its clients and partners in Russia "over the coming days," at which point "all transactions initiated with Visa cards issued in Russia will no longer work outside the country and any Visa cards issued by financial institutions outside of Russia will no longer work within the Russian Federation."

Mastercard said that cards issued by Russian banks "will no longer be supported by the Mastercard network," and those issued outside of the country "will not work at Russian merchants or ATMs."

Visa and Mastercard cards accounted for 74% of payment transactions in Russia in 2020, according to The Wall Street Journal.

Earlier this week, American Express announced it had halted relationships with bank partners in Russia impacted by U.S. and international sanctions.