While Russia launched massive missile strikes across Ukraine overnight, Ukrainian forces have claimed to have attacked a Russian military base on the occupied Crimean Peninsula.

A source in Ukraine's security services told ABC News on Thursday that Ukrainian forces had hit the Saki airfield in Moscow-annexed Crimea, using an initial wave of drones to "overload" Russian air defense. Russian air force assets were then struck using Neptune missiles designed and produced by Ukraine, according to the source.

Multiple unverified videos of the Ukrainian attack were circulating online Thursday.

-ABC News' Yulia Drozd, Oleksiy Pshemyskiy, Tatyana Rymarenko and Tom Soufi-Burridge

Russian forces launched missile strikes on at least five Ukrainian cities late Wednesday and early Thursday, just hours before Ukrainian President Volodymyr Zelenskyy's planned meeting with U.S. President Joe Biden in Washington, D.C.

Ukrainian Foreign Minister Dmytro Kuleba, who is traveling with Zelenskyy in the United States, described the strikes as "a massive missile attack" on civilian infrastructure, while Ukrainian state-owned grid operator Ukrenergo said it's the first major attack on the country's energy infrastructure in six months.

Russian forces fired a total of 43 missiles across Ukraine from east to west, and 36 of them were shot down by Ukrainian air defense, according to Gen. Valeriy Zaluzhny, commander-in-chief of the Ukrainian Armed Forces. The Ukrainian capital of Kyiv was among the major cities hit, along with areas of Kharkiv, Kherson, Cherkasy, Rivne and Lviv.

Ukrainian authorities were still assessing the damage and casualties on Thursday morning, but dozens of injuries have been reported so far. At least seven people were injured by falling debris in Kyiv.

Meanwhile, rescue efforts were ongoing in the central city of Cherkasy to evacuate as many as 20 people believed to be trapped beneath the rubble of a hotel that was destroyed in the strikes overnight. Thirteen others were already rescued and at least nine were injured, according to Ukrainian officials

The overnight strikes also targeted energy infrastructure in the Rivne region and an industrial zone in the Lviv area.

-ABC News' Victoria Beaulé, Guy Davies, Yulia Drozd and Tatyana Rymarenko.

Russian forces initiated six strikes on Kharkiv overnight, damaging civilian infrastructure, Ukrainian officials said early Thursday.

The mayor of Kyiv also said explosions occurred in the Ukrainian capital overnight. Debris from the downed rockets fell in the Darnytskyi and Holosiivskyi districts of the city.

Five people were hurt in the Darnytskyi district of Kyiv, where the strike also destroyed non-residential buildings. Three of them, including a 9-year-old girl, were hospitalized. Two were treated by medics on scene.

In the Shevchenkivskyi district of Kyiv, rocket debris damaged a gas pipe, an official said.

-ABC News' Will Gretsky

Seventeen people were killed, and 32 others injured, when a Russian missile hit a market in the center of Kostyantynivka in the Donetsk region, according to Ukrainian officials.

"At this moment, the artillery of Russian terrorists has killed 16 people in the city of Kostiantynivka, Donetsk region," President Volodymyr Zelenskyy said on Telegram. "A regular market. Shops. A pharmacy. People who did nothing wrong. Many wounded. Unfortunately, the number of casualties and the injured may rise."

U.S. President Joe Biden and other G-7 leaders are set to unveil future support for Ukraine on Wednesday after the conclusion of a high-stakes NATO summit in Lithuana's capital.

"President Biden and G-7 leaders will make a major announcement alongside President Zelenskyy this afternoon outlining our long-term commitments to support the people of Ukraine," U.S. National Security Council Senior Director for Europe Amanda Sloat said during a press briefing in Vilnius on Wednesday morning. "The United States, along with G-7 leaders, will announce our intent to help Ukraine build a military that can defend itself and deter a future attack."

"The launch of this process today will start a series of bilateral negotiations with Ukraine on the reaching of bilateral security commitments to help make this a reality," she added. "In particular, this process will ensure that the military assistance we provide Ukraine to defend itself against Russian aggression continues to be part of a long-term investment in Ukraine’s future force."

That investment will include making sure "Ukraine has a sustainable fighting force capable of defending Ukraine now and deterring Russian aggression in the future, a strong and stable economy, and the help Ukraine needs to advance the reform agenda to support the good governance necessary to advance Ukraine's Euro-Atlantic aspirations, which Ukraine recommits itself to as part of this declaration," according to Sloan.

"Taken together, we believe the declaration we will announce today seeks to ensure Ukraine's future as a free, independent, democratic, and sovereign nation," she told reporters. "This multilateral declaration will send a significant signal to Russia that time is not on its side."

-ABC News' Molly Nagle