The warning came as Russia issued a counter-claim saying it targeted soldiers and killed 200 Ukrainian servicemen in the attack Wednesday on a rail hub in Chaplyne city of the Dnipropetrovsk region.

The attack struck six months to the day since Russia began its invasion of Ukraine, which was also the day Ukraine celebrates its 1991 independence from the Soviet Union.

On Thursday, state rail operator Ukrainian Railways said the toll had risen overnight from 22 to 25, and included two children with a further 31 people injured.

Ukrainian President Volodymyr Zelensky had warned at the weekend Russia might do something "particularly cruel" to mark Ukraine's independence celebrations.

Moscow claimed to have slain over 200 Ukrainian troops and 10 units of military equipment in the attack.

In a daily press briefing, the defence ministry said the train was "en route to combat zones" in the eastern Donbas region of Ukraine, which Russia seeks to fully control.

But EU foreign policy chief Josep Borrell "strongly" condemned "another heinous attack by Russia on civilians".

"Those responsible for Russian rocket terror will be held accountable," he said on Twitter.

- 'Fight until the end' -

Washington warned Wednesday that Moscow was preparing to hold "sham" polls in occupied areas of Ukraine that would seek to formalise its control.

"Russian leadership has instructed officials to begin preparing to hold sham referenda," White House national security coordinator John Kirby said.

"We can see a Russian announcement of the first one or ones before the end of this week."

On Ukraine's Independence Day, messages of solidarity poured in from Ukraine's international allies.

British Prime Minister Boris Johnson paid a surprise visit to Kyiv, hailing Ukraine's staunch defence, as sirens sounded throughout the day.

Putin had failed to account for the "strong will of Ukrainians to resist", Johnson said.

"You defend your right to live in peace, in freedom, and that's why Ukraine will win," he added.

Earlier, Zelensky issued his own defiant morning video address, saying: "We don't care what army you have, we only care about our land. We will fight for it until the end."

Referring to Russia, he vowed Ukraine "will not try to find an understanding with terrorists".

"Ukraine is the whole of Ukraine," he said. "All 25 regions, without any concession or compromise."

- Fresh aid -

The US marked the occasion by announcing $3 billion in fresh military aid.

The new funding will help Kyiv acquire more materiel for its armed forces, locked in a grinding war of attrition with Russian troops in the east and south, with neither side advancing significantly.

Johnson unveiled his own pound54 million ($64 million) package of aid, including 2,000 "state-of-the-art drones" as well as anti-tank munitions.

Gatherings were banned in the capital Kyiv and Zelensky had urged citizens to be on guard against "Russian terror".

Nevertheless he and his wife marked a minute of silence for fallen Ukrainian soldiers and laid yellow and blue floral bouquets at a memorial in central Kyiv.

United Nations Secretary-General Antonio Guterres called the anniversary of the start of Russia's war in Ukraine a "sad and tragic milestone".

European Commission President Ursula von der Leyen said the EU has been standing with Ukraine "from the very beginning" and "will be for as long as it takes".

Even Alexander Lukashenko, the authoritarian leader of Belarus congratulated Ukraine on its Independence Day, comments dismissed by a spokesman for the Ukrainian presidency.

Belarus offered its territory as a staging ground for Russia's invasion.