At least 22 people died and dozens were wounded on Wednesday, 24 August, in the town of Chaplyne on Ukraine's independence day after Russia launched a deadly missile attack on a train station.
Vowing retribution, President Volodymyr Zelensky said in his evening address, "Chaplyne is our pain today."
"We will definitely make the occupiers bear responsibility for everything they have done. And we will certainly drive the invaders out of our land," he added.
Ukrainian Foreign Minister tweeted some images of the attack, saying, "Terrorist Russia keeps killing Ukrainian civilians."
"They will receive a response, a powerful response. I want to say that each day… this response will grow, it will get stronger and stronger," the president had said while addressing a news conference on Tuesday.
On Wednesday, the US announced a new security assistance package for Ukraine of about $3 billion to coincide with Ukraine's independence day.
"The United States of America is committed to supporting the people of Ukraine as they continue the fight to defend their sovereignty," President Joe Biden said in a statement announcing the package.
He is expected to talk to Zelenskyy on the phone on Thursday about the aid package.
(With inputs from Reuters.)