Russian offensive in Ukraine's Donetsk

Ukrainian defenders have fought desperately to withstand a major Russian offensive in the Donetsk region, with the enemy laying down heavy artillery fire to pave the way for ground forces to advance, a senior Ukrainian official says.

After Russian forces on Sunday took control of Lysychansk, the last bastion of Ukrainian resistance in Luhansk, Ukraine's military braced for an assault on Donetsk, with the cities of Sloviansk and Kramatorsk both in the Russian line of fire.

Donetsk and Luhansk comprise the Donbas, the industrialised eastern part of Ukraine that has seen the biggest battle in Europe for generations.

There was heavy fighting at the edge of the Luhansk region, its governor Serhiy Gaidai told Ukrainian television, saying Russian regular army and reserve forces had been sent there in an apparent effort to cross the Siverskiy Donets River.

"We are holding back the enemy on the border of Luhansk region and Donetsk region," Gaidai wrote on Telegram on Wednesday.

"The occupiers are suffering significant losses, as they themselves admit," said Gaidai, citing testimonies from Russian POWs and residents who had spoken to Russian soldiers in the fallen cities of Sievieroronetsk and Lysychansk.

Gaidai earlier said Russian forces were engaged in widespread shelling as they launched their assault on Donetsk.

Russia says it wants to wrest control of the entire Donbas from Ukraine on behalf of Moscow-backed separatists in two self-proclaimed people's republics.

On Tuesday, Russian forces struck a market and a residential area in Sloviansk, killing at least two people and injuring seven, local officials said.

A Reuters reporter at the scene saw yellow smoke billowing from an auto supplies shop, and flames engulfing rows of market stalls as firefighters tried to extinguish the blaze.

Donetsk regional governor Pavlo Kyrylenko said Sloviansk and nearby Kramatorsk had suffered heavy shelling overnight. "There is no safe place without shelling in the Donetsk region."

Lysychansk, once a city of a 100,000 people, lies in ruins. Buildings are scorched and holed by shells, cars up-ended and streets strewn with rubble, testament to the ferocity of the battle it endured.

Tatiana Glushenko, a 45-year-old Lysychansk resident, told Reuters there were people still sheltering in basements and bomb shelters, including children and elderly.

Glushenko said she didn't think she would be safe in other parts of Ukraine, so remained in Lysychansk with her family.

"All of Ukraine is being shelled: western Ukraine, central Ukraine, Dnipro, Kyiv, everywhere. So we decided not to risk our lives and stay here, at home at least," she added.

Luhansk governor Gaidai said Russian forces were pillaging Lysychansk and its twin city Sievierodonetsk.

"They are hunting down pro-Ukraine residents. They are making deals with collaborators, they are identifying apartments where servicemen lived, breaking in and taking clothing," he said.

"Everything is being destroyed. Entire book collections in Ukrainian. This is deja vu - 1939 with Nazi Germany."

Moscow ramped up its war rhetoric with Duma speaker Vyacheslav Volodin saying Ukraine had become a "terrorist state".

In another sign Russia is bracing for a long war, the Duma passed two bills in their first reading that would allow the government to oblige firms to supply the military and make staff work overtime to support the invasion.