The World Health Organization said on Sunday it had confirmed "several" attacks on health care facilities in Ukraine.

The attacks caused "multiple deaths and injuries," WHO Director-General Tedros Adhanom Ghebreyesus said on Twitter.

"Additional reports are being investigated," he said. "Attacks on healthcare facilities or workers breach medical neutrality and are violations of international humanitarian law."

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

More than 1 million refugees from Ukraine have fled to Poland since the Russian invasion began on Feb. 24, Poland's Border Guard said on Sunday.

"This is a million human tragedies, a million people banished from their homes by the war," the Border Guard said in a statement on Twitter, noting that its latest tally was from 8 p.m. local time.

Earlier Sunday, Filippo Grandi, the U.N. High Commissioner for Refugees, said more than 1.5 million people have fled Ukraine to multiple neighboring countries since the Russian invasion began, including Romania, Hungary, Moldova and Slovakia.

-ABC News' Christopher Donato and Christine Theodorou