Russia is preparing for the next stage of its offensive in Ukraine, a Ukrainian military official said, after Moscow said its forces would step up military operations in "all operational areas".

Russian rockets and missiles have pounded cities in strikes that Kyiv says have killed dozens in recent days.

"It is not only missile strikes from the air and sea," Vadym Skibitskyi, a spokesman for Ukrainian military intelligence, said on Saturday. "We can see shelling along the entire line of contact, along the entire front line. There is an active use of tactical aviation and attack helicopters.

"Clearly preparations are now underway for the next stage of the offensive.”

The Ukrainian military said Russia appeared to be regrouping units for an offensive toward Sloviansk, a symbolically important city held by Ukraine in the eastern region of Donetsk.

The British defence ministry said on Sunday that Russia was reinforcing its defensive positions across the areas it occupies in southern Ukraine after pressure from Ukrainian forces and pledges from Ukrainian leaders to force Russia out. 

Ukraine says at least 40 people have been killed in Russian shelling of urban areas in the last three days, as the war launched by Russian President Vladimir Putin on Feb. 24 intensifies.

Rockets hit the northeastern town of Chuhuiv in Kharkiv region on Friday night, killing three people including a 70-year-old woman and wounding three others, said regional Governor Oleh Synehubov. 

"Three people lost their lives, why? What for? Because Putin went mad?" said Raisa Shapoval, 83, a distraught resident sitting in the ruins of her home.

To the south, more than 50 Russian Grad rockets pounded the city of Nikopol on the Dnipro River, killing two people who were found in the rubble, said Governor Valentyn Reznichenko.

Moscow, which calls the invasion a "special military operation" to demilitarise its neighbour and root out nationalists, says it uses high-precision weapons to degrade Ukraine's military infrastructure and protect its own security. Russia has repeatedly denied targeting civilians.

Kyiv and the West say the conflict is an unprovoked attempt to reconquer a country that broke free of Moscow's rule with the break-up of the Soviet Union in 1991.

Russian Defence Minister Sergei Shoigu ordered military units to intensify operations to prevent Ukrainian strikes on eastern Ukraine and other areas held by Russia, where he said Kyiv could hit civilian infrastructure or residents, according to a statement from the ministry.

His remarks appeared to be a direct response to what Kyiv says is a string of successful strikes carried out on 30 Russian logistics and ammunitions hubs, using several multiple launch rocket systems recently supplied by the West.

The strikes are causing havoc with Russian supply lines and have significantly reduced Russia's offensive capability, Ukraine's defence ministry spokesperson said on Friday.

Russian-backed separatists said Ukraine had hit the town of Alchevsk, east of Sloviansk, with six U.S.-made high mobility artillery rockets on Saturday. The self-styled Luhansk People's Republic (LPR) said the strikes had killed two civilians and damaged a bus warehouse, health camp and apartments.

There was no immediate response from Ukraine, which says it only targets military infrastructure.

Reuters