Serhiy Gaidai, governor of the Luhansk region, said Russian forces attacked Sievierodentsk's industrial zone on Friday and also attempted to enter and blockade Lysychansk.
"There was an air strike at Lysychansk. Sievierodonetsk was hit by artillery," Gaidai said on Saturday on the Telegram messaging app, adding the Azot chemical plant in Sievierodonetsk and the villages of Synetsky and Pavlograd and others were shelled.
He made no mention of casualties at the Azot chemical plant and Reuters could not immediately verify the information.
Gaidai said 17 people were evacuated on Friday from Lysychansk by police officers, rescuers and volunteers.
Ukraine on Friday said its troops had been ordered to retreat from Sievierodentsk, a key battleground city, as there was very little left to defend after weeks of intense fighting.
"During the last (several) days, an operation was conducted to withdraw our troops," Kharatin Starskyi, the press officer of a National Guard brigade, said on Saturday.
Starskyi, who had been in Sievierodonetsk, told morning television that the flow of information about the withdrawal was delayed to protect troops on the ground. The retreat marks the biggest reversal for Ukraine since the loss of the southern port of Mariupol in May.
News of the withdrawal on Friday came four months to the day since Russian President Vladimir Putin sent tens of thousands of troops over the border, unleashing a conflict that has killed thousands, uprooted millions and reduced whole cities to rubble.
The latest Russian advances appeared to bring the Kremlin closer to taking full control of Luhansk, one of Moscow's stated war objectives, and sets the stage for Lysychansk to become the next main focus of fighting.
Russia invaded Ukraine on February 24, but abandoned an early advance on the capital Kyiv in the face of fierce resistance bolstered by Western arms.
Since then Moscow and its proxies have focused on the south and Donbas, an eastern territory made up of Luhansk and its neighbour Donetsk, deploying overwhelming artillery in some of the heaviest ground fighting in Europe since World War Two.
On Saturday, Russia again launched missile strikes on military and civilian infrastructure in the north near Ukraine's second-biggest city Kharkiv through to Sievierodonetsk in the east, said the General Staff of Ukraine's Armed Forces.
Several regional governors reported shelling attacks on towns across Ukraine on Saturday.
Russia denies targeting civilians, but Kyiv and the West say Russian forces have committed war crimes against civilians.
Ukraine on Friday again pressed for more arms, with its top general, Valeriy Zaluzhniy, telling his US counterpart in a phone call that Kyiv needed "fire parity" with Moscow to stabilise the situation in Luhansk.
On Friday, Ukraine's general staff said its troops had some success in the southern Kherson region, forcing the Russians back from defensive positions near the village of Olhine, the latest of several Ukrainian counter-assaults.
Russia says it sent troops into Ukraine to degrade its southern neighbour's military capabilities and root out people it called dangerous nationalists.
Ukraine, which says Russia has launched an imperial-style land grab, this week won new support from the West.
In a major sign of support, European Union leaders this week approved Ukraine's formal candidature to join the bloc - a decision that Russia said on Friday amounted to the EU's "enslaving" of neighbouring countries.
13°C