Two strikes hit in the Shevchenkivskiy district of the capital, Mayor Vitaliy Klitschko said on Telegram.

There was no immediate information about casualties, but a close aide to the UN chief sent a message to journalists confirming they were safe.

Ukrainian officials responded on Twitter to the attack on Kyiv, one of the boldest since Russian forces retreated from around the capital weeks ago.

"By this heinous act of barbarism Russia demonstrates once again its attitude toward Ukraine, Europe, and the world,” Foreign Minister Dmytro Kuleba said.

Mykhaylo Podolyak, a senior aide to Ukrainian President Volodymyr Zelenskiy, noted that Guterres had visited Moscow and had a meeting with Russian President Vladimir Putin earlier this week.

"The day before he was sitting at a long table in the Kremlin, and today explosions are above his head," Podolyak said.

Guterres earlier decried the "absurdity" of Moscow's war in Ukraine. The UN chief toured several towns just outside Kyiv -- including Bucha and Borodyanka -- where the corpses of civilians, some showing signs of torture, were found after Russian troops withdrew earlier this month.

The apparent evidence of atrocities has prompted calls from several countries, as well as the UN and the International Criminal Court (ICC), for investigations to determine whether war crimes were committed.

"I fully support the ICC and I appeal to the Russian Federation to accept, to cooperate with the ICC," Guterres said after visiting the two locations. "But when we talk about war crimes, we cannot forget that the worst of crimes is war itself. The war is an absurdity in the 21st century. The war is evil. There is no way a war can be acceptable in the 21st century.

Later on April 28, Ukraine’s prosecutor-general accused 10 Russian soldiers of being “involved in the torture of peaceful people” in Bucha. Iryna Venediktova said that the soldiers of the 64th Guards Motor Rifle Brigade of the Russian armed forces are suspected of "cruelty toward civilians and other war crimes."

In an interview with RFE/RL's Ukrainian Service on April 28, Podolyak said evidence of war crimes committed by Russian troops in Bucha and other towns near Kyiv is one reason peace talks are stalled and he sees no reason at present to hold a face-to-face meeting of the negotiating teams.

He also played down the possibility of talks between Zelenskiy and Putin, saying Ukraine will be ready for such a meeting "when the legal positions are ready," but that this won't be any time soon.

Britain's Ministry of Defense warned early on April 28 that Russia's Black Sea Fleet retained the ability to strike Ukrainian and coastal targets.

Despite losing the landing ship Saratov and the cruiser Moskva, Russia still has some 20 naval vessels, including submarines, in the Black Sea operational zone, the ministry said on Twitter.

In its daily intelligence report, the ministry said that Russia was temporarily unable to replace its lost vessels, as Turkey's Bosphorus Strait connecting the Black Sea with the Sea of Marmara and the Mediterranean "remains closed to all non-Turkish warships."

In Brussels, NATO Secretary-General Jens Stoltenberg said the alliance is ready to back Ukraine in its war against Russia even if the conflict stretches for years.

"We need to be prepared for the long term.... There is absolutely the possibility that this war will drag on and last for months and years," Stoltenberg told a youth summit on April 28, adding that the alliance will help Kyiv upgrade its Soviet-era weapons to modern Western military equipment.

The United States, NATO's biggest member, has already provided $3 billion worth of weapons to Ukraine since the start of Russia's unprovoked invasion.

In Washington, Biden announced on April 28 that he will request another $33 billion from Congress to support Ukraine, a move that represents a dramatic escalation of U.S. funding for the country. Biden said the spending is necessary "to support Ukraine in its fight for freedom" and would meet Ukraine's needs through September.

The request includes more than $20 billion for weapons, ammunition, and other military assistance, as well as $8.5 billion in direct economic assistance to the government and $3 billion in humanitarian and food security aid.

The assistance package is more than twice as large as the $13.6 billion in defense and economic aid enacted last month to assist Ukraine and Western allies that is now almost exhausted.

Russia has told the West to stop sending arms to Ukraine, saying large Western deliveries of weapons were inflaming the conflict.

Addressing lawmakers in St. Petersburg on April 27, Putin warned against foreign interference in Ukraine.

"If someone intends to intervene in the ongoing events from the outside, and create strategic threats for Russia that are unacceptable to us, they should know that our retaliatory strikes will be lightning fast," said Putin, according to video of his address supplied by Russian media.

With reporting by AP, AFP, BBC, CNN, dpa, and Reuters