A Russian bomb has hit a blood transfusion centre in the Ukrainian region of Kharkiv, Volodymyr Zelenskyy has claimed.
On Telegram, the Ukrainian president said there are fatalities and injuries after the strike in the town of Kupiansk.
Mr Zelenskyy said rescue workers were extinguishing fires at the scene, and he described the attack as a “war crime”.
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Kupiansk is a railway hub that is fewer than 10 miles from the frontline – and at this point, it is unclear how many people have been killed or wounded.
Russia has previously denied targeting civilians in a full-scale invasion that has killed thousands of people, uprooted millions and destroyed cities.
Throughout Saturday, explosions and air alerts were reported across Ukraine – including in the capital of Kyiv.
It comes after Russia’s defence ministry warned of retaliation following drone attacks on ships in the Black Sea.
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Watch: Ukrainian sea drone attacks Russian ship
A tanker transporting fuel for Putin’s forces was hit on Friday night by a sea drone that was filled with 450kg (992lbs) of TNT – and a major port was targeted earlier in the day.
“There can be no justification for such barbaric actions, they will not go unanswered and their authors and perpetrators will inevitably be punished,” spokeswoman Maria Zakharova said.
In other developments, Ukraine has reportedly used cluster monitions in the Russian-occupied city of Donetsk – with a university on fire.
The controversial weapons are banned in more than 120 countries because they have a track record of killing and maiming civilians.
They detonate in the air and releases “bomblets” that scatter over a large area, but smaller munitions can fail to detonate and pose a long-term risk to civilians.
Cluster bombs have been supplied to Ukrainian forces by the US.
President Joe Biden said the controversial decision was a “difficult” one – but he had to act because Mr Zelenskyy’s forces were running out of ammunition.