Russian forces on Wednesday launched offensives on towns in eastern Ukraine with constant mortar bombardment, destroying several houses and killing civilians, Ukrainian officials said, as Russia focuses its attack on the industrial Donbass region.

Russia is attempting to seize the separatist-claimed Donbass' two provinces, Donetsk and Luhansk, and trap Ukrainian forces in a pocket on the main eastern front.

In the easternmost part of the Ukrainian-held Donbass pocket, the city of Severodonetsk on the east bank of the Siverskiy Donets River and its twin Lysychansk, on the west bank, have become a pivotal battlefield. Russian forces were advancing from three directions to encircle them.

Ukrainian President Volodymyr Zelenskyy's office said Russian forces launched an offensive on Severodonetsk early Wednesday and the town was under constant fire from mortars.

Zelenskyy mourned the thousands of Ukrainian men and women who have perished since the start of the Russian invasion while renewing calls for heavy weapons from foreign partners, saying arms for Kyiv were "the best investment in stability in the world."

In his daily address to the nation late Tuesday, Zelenskyy called the situation in Donbass "extremely difficult."

"All the strength of the Russian army they still have was thrown there for the offensive," Zelenskyy said. "The occupiers want to destroy everything there."

Supplying Ukraine with rocket-propelled grenades, tanks, anti-ship missiles and other weapons is "the best investment" in preventing future Russian aggression, Zelenskyy said.

"The longer this war lasts, the greater will be the price of protecting freedom, not only in Ukraine, but also in the whole free world."

Luhansk regional Governor Serhiy Gaidai said six civilians were killed and at least eight wounded, most near bomb shelters, in Severodonetsk.

"At the moment, with the support of artillery, the Russian occupiers are attacking Severodonetsk," Gaidai said.

Ukraine's military said it had repelled nine Russian attacks on Tuesday in Donbass, where Moscow's troops had killed at least 14 civilians, using aircraft, rocket launchers, artillery, tanks, mortars and missiles.

Reuters could not immediately verify information about the fighting.

In a sign of Ukrainian success elsewhere, authorities in the country's second-largest city, Kharkiv, reopened the underground metro, where thousands of civilians had sheltered for months under relentless bombardment.

The reopening came after Ukraine pushed Russian forces largely out of artillery range of the northern city, as they did from the capital, Kyiv, in March.

World War III?

Three months into the invasion, Russia still has only limited gains to show for its worst military losses in decades, while much of Ukraine has suffered devastation in the biggest attack on a European state since 1945.

More than 6.5 million people have fled abroad, uncounted thousands have been killed and cities have been reduced to rubble. The war has also caused growing food shortages and soaring prices due to sanctions and disruption of supply chains. Both Ukraine and Russia are major exporters of grain and other commodities.

European Commission chief Ursula von der Leyen accused Russia of using food as a weapon.

Billionaire financier George Soros, speaking in Davos, said Russia's invasion of Ukraine may have marked the start of World War III.

"The best and perhaps only way to preserve our civilization is to defeat Putin as soon as possible," he said.

Jailed Russian opposition leader Alexei Navalny on Tuesday lambasted President Vladimir Putin, casting the Kremlin chief as a doomed madman who was butchering the people of both Ukraine and Russia.

"This is a stupid war which your Putin started," Navalny told an appeals court in Moscow via video link from a corrective penal colony. "This war was built on lies."

Underlining the global tensions unleashed by the war, major U.S. ally Japan scrambled jets on Tuesday after Russian and Chinese warplanes neared its airspace as U.S. President Joe Biden visited Tokyo.

On Monday, Starbucks Corp. became the latest Western brand to announce it was pulling out of Russia, following a similar decision by McDonald's. The hamburger chain's trademark "Golden Arches" was lowered near Moscow on Monday.

Drawn out conflict

Senior Russian officials suggested in comments on Tuesday the war, which Russia calls a "special operation," may be drawn-out.

Nikolai Patrushev, head of Putin's security council, said Russia would fight as long as necessary to eradicate "Nazism" in Ukraine, a justification for the war that the West calls baseless.

Defense Minister Sergei Shoigu said Russia was deliberately advancing slowly to avoid civilian casualties.

Zelenskyy dismissed such statements as "absolutely unreal."

In Kharkiv, hundreds of people were living underground in trains and stations when the authorities asked them to make way out on Tuesday.

The Donbass fighting follows Russia's biggest victory in months: the surrender last week of Ukraine's garrison in the port of Mariupol after a siege in which Kyiv believes tens of thousands of civilians were killed.

Petro Andryushchenko, an aide to Mariupol's Ukrainian mayor now operating outside the city, said the dead were lying in the rubble.

About 200 decomposing bodies were buried in debris in the basement of one high-rise building, he said. Residents had refused to collect them and Russian authorities had abandoned the site.