"Provisional toll from today's shelling: six dead, 15 injured. Among the dead, there is a child," Vadim Lyakh said on Facebook, confirming a toll given by a spokeswoman for the Donetsk region to Ukrainian media.

Lyakh had earlier described the attack on Sloviansk as "the heaviest for a long time", saying there were 15 fires and "many dead and wounded".

Tetyana Ignachenko, a spokeswoman for the Donetsk region to which Sloviansk belongs, reiterated a call on residents to leave the region, as the frontline is just a few kilometres (miles) away.

Ukrainian media said one of the city's markets was on fire following the strikes.

Further south, the city of Kramatorsk, which is the administrative centre of Ukraine's eastern Donbas region, was hit for the second day in a row by Smerch rockets, according to its mayor Oleksandr Goncharenko.

The strikes, which hit a residential area and an unoccupied hotel, caused no casualties, he said.

Sloviansk and Kramatorsk are key targets for Moscow, particularly if the capture of Lysychansk in the northeast claimed by Russia on Sunday is confirmed.