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.

U.S. Secretary of State Antony Blinken crossed the border into Ukraine Saturday after meeting with Ukrainian Foreign Minister Dmytro Kuleba at the Korczowa Border Crossing Point.

"The entire world stand with Ukraine," Blinken said, and Kuleba added that he hopes Ukrainians will see this as a "clear manifestation that we have friends who literally stand by us."

Blinken said the sanctions imposed so far against Russia are "producing very, very concrete results" and that the pressure will "grow" until this "war of choice is brought to an end."

When asked directly about helping Kuleba with more firepower, Blinken said, "the support for Ukraine not only has been unprecedented, not only is going to continue, it's going to increase."

Kuleba said Ukraine "appreciates" sanctions that have been announced in the last week, but that more economic and political pressure and "necessary weapons," would "save many lives in Ukraine... many sufferings will be avoided."

Kuleba also said that they are “satisfied" with already arranged supplies of anti-tank weapons and ammunition, but that “it’s no secret” they still want fighter jets and air defense systems.

He also thanked the U.S. for sending stingers, and said they were used on Saturday.

"Just today we shot down three Russian attack aircrafts, which were bombing our cities with the use of stingers but we need -- so to say -- big air defense systems to ensure the safety of our skies. If we lose the skies, there will be much, much more blood on the ground,"Kuleba said.

In response to questions about NATO’s refusal to impose a no-fly zone over Ukraine, Kuleba said it’s a "sign of weakness," but stayed optimistic that they will change their minds.

"The time will come," he said.

-ABC News' Conor Finnegan and Justin Gomez

The U.S. Department of Commerce announced Friday that it will restrict the export of U.S. luxury goods to Russia and Belarus, as well as "certain Russian and Belarusian oligarchs and malign actors located worldwide," as a result of their actions in Ukraine.

The Department of Commerce said it will impose restrictions on the export, reexport and transfer of luxury items including certain spirits, tobacco products, clothing items, jewelry, vehicles and antique goods.

"Putin’s war of choice in Ukraine continues to take a devastating toll on innocent civilians in Ukraine, fueling one of the worst humanitarian crises Europe has seen in decades," Secretary of Commerce Gina Raimondo said in a statement.

Raimondo added, "Putin and the oligarchs who fund him have gotten rich off of Putin’s rampant corruption and the exploitation of the Russian people. We will not allow Putin and his cronies to continue living in opulence while causing tremendous suffering throughout Eastern Europe. Today’s action takes away another source of comfort and reminds them that Russia is increasingly isolated."

-ABC News' Luke Barr