US Secretary of State Antony Blinken announced $1 billion of aid during a surprise visit Wednesday to Ukraine, which suffered a Russian strike that killed at least 17 people at a market.
The attack, which President Volodymyr Zelensky described as deliberate and "heinous", drew international condemnation from the West, including accusations of war crimes.
Projectiles tore through the centre of Kostiantynivka -- a town of nearly 70,000 people in the eastern Donetsk region -- in one of the deadliest strikes in weeks.
"They smashed everything, all the shop windows, everything was strewn around," an eyewitness told AFP.
"Thank God we are alive, of course. But the girls who were selling there, they are all dead," the witness said.
Images distributed by officials showed rescue workers picking through the debris and carrying away some of the 32 people reportedly wounded in the blast, which left vehicles charred and kiosks torn to pieces.
"Anyone in the world who is still dealing with anything Russian simply ignores this reality," said Zelensky.
"Heinous evil. Brazen wickedness. Utter inhumanity."
He later accused Russia of deliberately targeting civilians and said there were no military units "anywhere near" the scene.
The European Union condemned the strike along with the "escalation" of Russian attacks on "civilian objects" that has seen hundreds killed or wounded in recent weeks.
"Intentional attacks against civilians are war crimes," the bloc said in a statement.
Germany's Foreign Minister Annalena Baerbock wrote on X, formerly Twitter, that "this Russian war of aggression is an attack on international law, on humanity."
Meanwhile, White House spokeswoman Karine Jean-Pierre said the incident underscored "the importance of continuing to support the people of Ukraine as they defend their territory."
- 'Build momentum' -
During a meeting with Zelensky, Blinken reiterated Washington's support for Kyiv in its fight to liberate territory in the south and east.
"We are determined in the United States to continue to walk side by side with you. And President Biden asked me to come to reaffirm strongly our support," he told Zelensky.
"We see the important progress that's being made now in the counteroffensive and that's very, very encouraging," he added.
The new $1 billion aid package, which includes $665.5 million in military and civilian security assistance, would further "build momentum" for the counteroffensive, Blinken said at a later press conference.
In addition to the funds, the Pentagon announced it will provide Ukraine with depleted uranium tank ammunition -- a powerful but controversial weapon due its toxicity.
Russia's embassy in the United States said on Telegram that the move was "a clear sign of inhumanity" on Washington's part.
"The US is deliberately transferring weapons with indiscriminate effects," it said.
The Kremlin earlier dismissed Blinken's visit, arguing US aid would not "influence the course of the special military operation" -- Moscow's term for its offensive.
Kyiv's army meanwhile said it was pressing on with "offensive operations" towards eastern Ukraine's war-battered town of Bakhmut, which fell to Russian forces in May, and the Moscow-occupied southern city of Melitopol.
The boost in US aid to Kyiv follows criticisms in recent weeks that Ukraine's counteroffensive has been too slow.
Russia said Wednesday it had "improved its tactical position" near the northwestern city of Kupiansk, where it has led a local offensive for weeks.
It earlier hit Ukraine's southwestern Odesa region, near the border with Romania, with overnight drone attacks killing one person.
Early Thursday, Russia's defence ministry said three Ukrainian drones had been destroyed during the night: one on the outskirts of Moscow and two over the southwestern Rostov region.
"Several cars were damaged" in the centre of Rostov, along with building facades and windows, regional governor Vasily Golubev said on Telegram.
One person sustained cuts but declined hospital treatment, he added.
The Russian capital's Mayor Sergei Sobyanin said on Telegram that debris from the "drone attack attempt on Moscow" caused no damage or casualties, according to initial reports.
- 'Everything possible and impossible' -
Ukrainian lawmakers approved Wednesday the nomination of Crimean Tatar Rustem Umerov as Kyiv's new wartime defence minister, in what was hailed as a historic move.
Crimean Tatars are an ethnic minority hailing from the Black Sea peninsula, which was annexed by Russia in 2014.
"I will do everything possible and impossible for the victory of Ukraine -- when we liberate every centimetre of our country and every one of our people," he said on social media.
The 41-year-old businessman has been involved in prisoner exchange negotiations involving Saudi Arabia and grain export talks with Turkey and the United Nations.
Zelensky had nominated Umerov as new defence minister after the resignation of Oleksiy Reznikov, calling for "new approaches" in the wake of several corruption scandals within the ministry.
"It is the highest state post ever held by a Tatar (from Crimea)," Sergiy Leshchenko, an advisor to the presidential administration, told AFP.
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