More than 30 people were killed and over 100 were wounded in a Russian rocket strike on a railway station packed with women, children and elderly evacuees in east Ukraine early on Friday, Ukrainian officials said.

Two powerful rockets struck the station in the city of Kramatorsk in what President Volodymr Zelenskiy said was a deliberate attack on civilians.

"Lacking the strength and courage to stand up to us on the battlefield, they are cynically destroying the civilian population," Zelenskiy said after the state railway company issued the preliminary casualty toll of over 30 dead.

"This is an evil that has no limits. And if it is not punished, it will never stop."

Reuters could not verify what happened at the station.

The Russian defence ministry was quoted by RIA news agency as saying the missiles said to have struck the station were used only by Ukraine's military and that Russia's armed forces did not have any targets assigned in Kramatorsk on Friday.

The head of Ukrainian Railways said at least two children were among the dead. Kramatorsk Mayor Oleksander Honcharenko said about 4,000 people had been at the station at the time of the attack.

Pavlo Kyrylenko, governor of the Donetsk region, published a photograph online showing several bodies on the ground beside piles of suitcases and other luggage. Armed police wearing flak jackets stood beside them.

Another photo showed rescue services tackling what appeared to be a fire, with a pall of grey smoke rising into the air.

"The 'Rashists' ('Russian fascists') knew very well where they were aiming and what they wanted: they wanted to sow panic and fear, they wanted to take as many civilians as possible," he wrote in an online post.

Reuters could not immediately verify the photos.

CROWDED STATION

"They (Russian forces) wanted to hit the station," Mayor Honcharenko said, a view shared by presidential adviser Oleksiy Arestovych.

"It must be understood that such strikes are preceded by a thorough reconnaissance of the target, at least by drones, gunners on the ground - it's too expensive a missile and too difficult and risky to organise such strikes," Arestovych said.

"They (Russian forces) could clearly see that they were striking civilians early in the morning, that there were thousands of people trying to evacuate at the station at that time - families, children, the elderly."

Three trains carrying evacuees were blocked in the same region of Ukraine on Thursday after an air strike on the line, according to the head of Ukrainian Railways.

Ukrainian officials say Russian forces have been regrouping for a new offensive, and that Moscow plans to seize as much territory as it can in the eastern part of Ukraine known as Donbas bordering Russia.

Local authorities in some areas have been urging civilians to leave while it is still possible, and relatively safe, to do so.

Russia denies targeting civilians in "special military operation" aimed at demilitarizing and "denazifying" Ukraine. The Kremlin's position is rejected by Ukraine and the West as a pretext for an unprovoked invasion.