Russia resumed its assault on the last Ukrainian defenders holed up in a giant steel works in Mariupol, a Ukrainian official said on Saturday, days after Moscow declared victory in the southern port city and said its forces did not need to take the factory.

Russian forces were hitting the Azovstal complex with air strikes and trying to storm it, presidential adviser Oleksiy Arestovych said, adding "the enemy is trying to strangle the final resistance of Mariupol's defenders".

The biggest battle of the conflict has raged for weeks as Russia seeks to capture a city seen as crucial to its attempts to link the eastern Donbas region with Crimea, the Black Sea peninsula that Moscow seized in 2014.

Russia's defence ministry said on Friday the remaining fighters had been "securely blockaded" at the steel plant. On Thursday, President Vladimir Putin had declared the city "liberated" and ordered his defence minister to block off the Azovstal complex "so not even a fly can get through" rather than try to storm it.

"There's no need to climb into these catacombs and crawl underground through these industrial facilities" Putin said on television.

Arestovych said Ukrainian troops in the Azovstal complex were still holding out "despite the very difficult situation" and were attempting counterattacks.

More than 1,000 civilians are holed up with the troops at the Azovstal plant, according to Ukrainian authorities.

Russian forces besieged and bombarded Mariupol - home to more than 400,000 people before the war - for weeks, leaving a city in ruins. Ukraine estimates tens of thousands of civilians have died and says 100,000 civilians are still there. The United Nations and Red Cross say the civilian toll is at least in the thousands.

Maxar Technologies, a U.S. company, said on Friday that satellite imagery from near Mariupol showed a second cemetery had been expanded in late March and early April at Vynohradne, with long new trenches likely to become new grave sites.

The company had said on Thursday its imagery had located a separate burial site in another location near the city with more than 200 graves.

Russia has said the invasion it launched on Feb. 24 is a "special military operation" to demilitarise Ukraine and liberate its population from dangerous nationalists. Moscow has denied targetting civilians. Ukraine and its Western allies call the invasion an unjustified war of aggression.

On Saturday Luhansk governor Serhiy Gaidai said Ukrainian forces were pulling back from some settlements to new defensive lines to preserve their units in the face of an intensifying barrage on all cities in the region. "It's unpleasant they're leaving our settlements, but it is no catastrophe," Gaidai added.

Russia's current offensive is focused on the Donbas, where Moscow-backed separatists have for years held parts of its constituent Donetsk and Luhansk regions.

The governor of Kharkiv, a heavily bombarded city northwest of Donbas, said Russian forces had carried out over 50 artillery or rocket attacks in the area over the past day, killing two people and wounding 19.

Governor Oleh Synegubov said intense fighting continued at the frontline town of Izyum, southeast of the city, and that Ukrainian forces had retaken three villages to its northwest.

Russia said on Saturday it had shot down a Ukrainian fighter jet and destroyed three Ukrainian helicopters at an airfield in Kharkiv. There was no immediate reaction from Ukraine regarding the Russian claims but the Ukrainian military said on Saturday it had destroyed 177 Russian aircraft and 154 helicopters since the start of the war.

Reuters could not independently verify the claims. Russian forces made no major gains in the last 24 hours, British military intelligence said on Saturday.

On Friday, Russian general Rustam Minnekayev said Moscow wanted control not just of the Donbas but also over southern Ukraine, suggesting wider goals for the invasion than Russia has previously acknowledged. Minnekayev, deputy commander of Russia's central military district, was quoted by Russian state news agencies on Friday as saying full control over southern Ukraine would give it access to Transdniestria, a breakaway Russian-occupied part of Moldova in the west.

That would cut off Ukraine's entire coastline and mean Russian forces pushing hundreds of miles further west, past the major coastal city of Odesa. Ukrainian President Volodymyr Zelenskiy said after Minnekayev's comments that Russia's invasion was just the beginning and that Moscow has designs on capturing other countries.

"We are the first in line. And who will come next?" he said in a video address late on Friday. Kremlin spokesman Dmitry Peskov declined to comment on whether Russia had expanded its goals or on how Moscow saw the political future of southern Ukraine. Zelensky also said in his address that Ukraine's allies were finally delivering weapons Kyiv has asked for.

US. President Joe Biden said on Thursday he had authorized a further $800 million in military aid for Ukraine, including heavy artillery, ammunition and drones. Canada said on Friday it had provided more heavy artillery to Ukraine.