Russian missiles hit the heart of the central Ukrainian city of Vinnytsia on Thursday morning, killing at least 23 people and wounding dozens, according to Ukraine's State Emergency Service.

Three children were among the dead, the agency said.

The missiles struck an office building and damaged nearby residential buildings in Vinnytsia, located about 155 miles southwest of the Ukrainian capital, Kyiv. The strike also ignited a massive fire that engulfed 50 cars in an adjacent parking lot, according to the National Police of Ukraine. Burned-out vehicles are peppered with holes from the missiles.

The State Emergency Service said about 115 victims in Vinnytsia needed medical attention, with 64 people hospitalized -- including 34 in severe condition and five in critical.

Forty-two people are listed as missing, the agency said.

Many Ukrainians moved to Vinnytsia, a city southwest of Kyiv, to get away from the fighting in eastern Ukraine. Until now, Vinnytsia had been seen as a city of relative safety.

Ukrainian President Volodymyr Zelenskyy called the attack "an open act of terrorism" on civilians.

"Every day Russia is destroying the civilian population, killing Ukrainian children, directing missiles at civilian objects. Where there is no military (targets). What is it if not an open act of terrorism?” Zelenskyy said in a statement via Telegram on Thursday.

War crimes investigators are at the scene studying missile fragments.

Russian missile strikes targeted several other Ukrainian cities on Wednesday and early Thursday, including Kharkiv, Zaporizhia and Mykolaiv.

At least 12 people died in the Zaporizhia strike, which hit two industrial workshops on Wednesday, according to local authorities.

At least five civilians were killed and 30 others injured in Mykolaiv on Wednesday after Russian missiles destroyed a hotel and a shopping mall, the local mayor said. The southern Ukrainian city was shelled again on Thursday morning, but no casualties were immediately reported.

-ABC News' Edward Szekeres, Fidel Pavlenko, Max Uzol, and Yulia Drozd

Michael Carpenter, the U.S. ambassador to the Organization for Security and Cooperation in Europe, is calling the forcible relocation of Ukrainians to Russian filtration camps is "a war crime."

In an interview with ABC News Live on Thursday, Carpenter said the Russians are "trying to take away Ukrainians who might have Ukrainian civic impulses, who are patriots, who want to defend their country." Carpenter said the Russians want to "erase Ukrainian identity" and "the Ukrainian nation state, as the entity that governs people's lives in these regions."

Carpenter said there are at least 18 filtration camps along the Russia-Ukraine border, adding that it’s impossible to get an exact total because many are located in far-flung locations across Russia, including the far east.

-ABC News' Malka Abramoff

The State Department said Wednesday it is aware of unconfirmed reports that another American has been detained by pro-Russian separatists in Ukraine.

The statement follows a [report from the Guardian] () on 35-year-old Suedi Murekezi, who is believed to have gone missing in Ukraine in early June.

According to the Guardian, Murekezi was able to make contact with a family member on July 7 and told them he was being held in the same prison as Alexander Drueke and Andy Tai Ngoc Huynh, two American veterans captured while volunteering for Ukrainian forces. Murekezi has lived in Ukraine since 2020 and was falsely accused of participating in pro-Ukraine protests, according to the report.

"We have been in contact with the Ukrainian and Russian authorities regarding U.S. citizens who may have been captured by Russia's forces or proxies while fighting in Ukraine," a State Department spokesperson said Wednesday. "We call on Russia to live up to its international obligations to treat all individuals captured fighting with Ukraine's armed forces as prisoners of war."

Another American -- Grady Kurpasi -- is also missing in Ukraine. A family spokesperson said the veteran was last seen fighting with Ukrainian forces in late April and is feared to have been either killed or captured.

-ABC News' Shannon Crawford

Shelling from both Russian and Ukrainian forces caused damage to the landscape and destroyed structures throughout the Donbas region on Tuesday and Wednesday, local officials said.

Russian strikes reportedly targeted the eastern town of Bakhmut, killing one person and wounding 5 others, the local governor said. Explosions were heard in several nearby towns too, with one missile falling near a kindergarten.

Shelling also continued in Izyum, Mykolayiv and Kharkiv on Tuesday. Russian troops reportedly conducted unsuccessful attacks north of Slovyansk and the town of Siversk on Tuesday, despite repeated rhetoric of an “operational pause” that Russia allegedly maintains, the Institute for the Study of War said in its latest report.

Russian forces continue to bomb critical areas in preparation for future ground offensive, with air and artillery strikes reported along the majority of the frontline, the experts added.

Ukrainian forces on Tuesday responded to the Russian attacks and claimed to have destroyed six Russian military facilities on occupied Ukrainian territories. Ukrainian officials claimed to have destroyed several ammunition depots, as well as a larger military unit.

Russian media reported on Tuesday that Ukrainian troops launched a “massive attack” on an air defense unit in the Luhansk region.

Ukrainian military officials also claimed to have killed at least 30 Russian troops on Tuesday, along with destroying a howitzer and a multiple rocket launcher, among other weaponry.

But the U.K. Defense Ministry in its latest intelligence update said it still expects Russian forces to “focus on taking several small towns during the coming weeks” in the Donbas region.

These towns are on the approaches to the larger cities of Slovyansk and Kramatorsk that likely remain the principal objectives for this phase of the Russian military operation, the ministry said.

-ABC News' Edward Szekeres, Max Uzol, Yulia Drozd and Yuriy Zaliznyak

The first ship carrying Ukrainian grain departed Odesa on Monday morning under an internationally brokered deal attempting to ease a global hunger crisis.

The Sierra Leone-flagged cargo ship Razoni left the Ukrainian port city and is headed to Lebanon, a tiny Mideast nation that imports nearly all of its grain and lacks storage space after a 2020 explosion destroyed grain silos at its main port in Beirut. The vessel is expected to reach Istanbul on Tuesday, where it will be inspected before being allowed to proceed to Tripoli, according to a statement from the Turkish Ministry of National Defense.

Razoni, which is carrying 26,527 tons of corn, is the first commercial ship to set off from Ukraine's port of Odesa since Feb. 26 and the first vessel to depart under the so-called Black Sea Grain Initiative, according to a statement from the spokesperson for the the United Nations secretary-general. Last month, Russia and Ukraine signed separate agreements with Turkey and the U.N. to allow Ukraine to resume its shipment of grain from the Black Sea to world markets and for Russia to export grain and fertilizers.

Since Russian forces invaded neighboring Ukraine on Feb. 24, the cost of grain, fertilizer and fuel has skyrocketed worldwide. Russia and Ukraine -- often referred to collectively as Europe's breadbasket -- produce a third of the global supply of wheat and barley, but a Russian blockade in the Black Sea combined with Ukrainian naval mines have made exporting siloed grain and other foodstuffs virtually impossible. As a result, millions of people around the world -- particularly in Africa and the Middle East -- are now on the brink of famine.