A Ukrainian regional official has warned of deteriorating living conditions in a city captured by Russian forces two weeks ago, saying Sievierodonetsk is without water, power or a working sewage system while the bodies of the dead decompose in hot apartment buildings.

Governor Serhiy Haidai said the Russians are unleashing indiscriminate artillery barrages as they try to secure their gains in eastern Ukraine's Luhansk province.

Moscow this week claimed full control of Luhansk, but the governor and other Ukrainian officials said their troops retained a small part of the province.

"Luhansk hasn't been fully captured even though the Russians have engaged all their arsenal to achieve that goal," Mr Haidai told The Associated Press.

"Fierce battles are going on in several villages on the region's border. The Russians are relying on tanks and artillery to advance, leaving scorched earth."

Russia's forces "strike every building that they think could be a fortified position", he said.

"They aren't stopped by the fact that civilians are left there and they die in their homes and courtyards. They keep firing."

Occupied Sievierodonetsk, meanwhile, "is on the verge of a humanitarian catastrophe", the governor wrote on social media.

"The Russians have completely destroyed all the critical infrastructure, and they are unable to repair anything."

Mr Haidai reported last week that about 8,000 residents remained in the city, which had a pre-war population of around 100,000.

Some Ukrainian officials and soldiers said Russian forces levelled Sievierodonetsk, Luhansk province's administrative centre, before Ukraine's troops were ordered out of the city late last month to avoid their encirclement and capture.

Luhansk is one of two provinces that make up the Donbas, a region of mines and factories where pro-Moscow separatists have fought Ukraine's army for eight years and declared independent republics that Russian President Vladimir Putin recognised before he sent troops into Ukraine.

After asserting full control of Luhansk, Mr Putin said Russian forces would have a chance to rest and recoup, but other parts of eastern Ukraine have come under sustained bombardment.

The Russian leader warned Kyiv it should quickly accept Moscow's terms or brace for the worst.

"Everybody should know that largely speaking, we haven't even yet started anything in earnest," Mr Putin said while speaking with leaders of the Kremlin-controlled parliament Thursday.

Ukraine's presidential office said on Friday that at least 12 civilians were killed and another 30 wounded by Russian shelling over the previous 24 hours.

Two cities in Donetsk - the other Donbas province - experienced the heaviest barrage, with six people killed and 21 wounded.

In north-east Ukraine, another four people were killed and nine were wounded in Kharkiv, the country's second-largest city, where Russian shelling hit residential areas.

Commenting on Mr Putin's ominous statement, Kremlin spokesman Dmitry Peskov said the Russian leader was reacting to statements by Ukraine's government and its western allies about defeating Russia on the battlefield.

"Russia's potential is so big that just a small part of it has been used in the special military operation," Mr Peskov told reporters.

"And so western statements are utterly absurd and just add to the grief of the Ukrainian people."

In other developments:

- Germany's parliament overwhelmingly approved Sweden and Finland's requests to join Nato. German defence minister Christine Lambrecht said the two countries' accession would greatly strengthen Nato's northern and eastern flanks, noting their strong naval forces in the Baltic Sea and their land forces that know the region bordering Russia well. She suggested that Mr Putin's efforts to divide and destroy Nato had failed. "He bet on our weakness," she said. "Now he gets the opposite." All 30 member countries must agree before the western military alliance can admit Finland and Sweden.

- A court in Moscow sentenced a Russian municipal council member who had publicly criticised the war in Ukraine to seven years in prison on charges of "knowingly false information" about the Russian military. Alexei Gorinov, 60, criticised Russia's military actions in Ukraine at a March meeting. A legal aid group said he is the first person ordered to serve prison time under a law that makes it illegal to disparage the Russian military. Russia's parliament rubber-stamped the law, which carries a maximum prison term of 15 years, a week after the Kremlin sent troops into Ukraine.

- The British Ministry of Defence said Ukrainian forces made advances near the Russian-occupied southern city of Kherson. The ministry's daily intelligence briefing mentioned the counter-offensive as Ukrainian partisan activity also targets Russian forces in southern Ukraine. The Washington-based Institute for the Study of War said partisans blew up a railway bridge some 15 miles north of Melitopol, which is to the east of Kherson, on Thursday to disrupt Russian resupply operations.