(AFP) - Ukraine marked 100 days since Russia's invasion on Friday with fighting raging across the east of the country, where Moscow's forces are tightening their grip on the Donbas region.

The sombre milestone came as Kyiv announced Moscow was now in control of a fifth of Ukrainian territory, including Crimea and parts of Donbas seized in 2014.

After being repelled from around the capital, President Vladimir Putin's troops have set their sights on capturing eastern Ukraine, prompting warnings the war could drag on.

Some of the fiercest fighting is now centred on Severodonetsk in the Donbas region, 80 percent of which the Russians have seized, but Ukrainian forces are putting up stiff resistance.

President Volodymyr Zelensky said late Thursday Ukrainian forces had had some success in the battle for the industrial hub, which is in the Lugansk region.

"But it is still too early. It is the toughest area at the moment," he added.

Lugansk regional governor Sergiy Gaiday said on Telegram that "for 100 days, they have been levelling everything", accusing the Russians of destroying hospitals, schools and roads.

"But we are only getting stronger. Hatred of the enemy and faith in our victory make us unbreakable."

Since Russia's February 24 invasion, thousands of people have been killed and millions forced to flee, while Zelensky says up to 100 Ukrainian soldiers are dying every day on the battlefield.

Severodonetsk's Azot factory, one of Europe's biggest chemical plants, was targeted by Russian soldiers who fired on one of its administrative buildings and a warehouse where methanol was stored.

- 'Shelling getting stronger' -

Ukrainian troops were still holding an industrial zone, Gaiday said, a situation reminiscent of Mariupol, where a huge steelworks was the southeastern port city's last holdout until Ukrainian troops finally surrendered in late May.

The situation in Lysychansk -- Severodonetsk's twin city, which sits just across a river -- also looked increasingly dire.

About 60 percent of infrastructure and housing had been destroyed, while internet, mobile network and gas services had been knocked out, said the city's mayor Oleksandr Zaika.

"The shelling is getting stronger every day," he said.

In the city of Sloviansk, about 80 kilometres (50 miles) from Severodonetsk, residents said there were constant bombardments by Russian troops.

"It's very difficult here," said paramedic Ekaterina Perednenko, 24, who only returned to the city five days ago but realises that she will have to leave again.

"Shooting is everywhere, it's scary. No water, electricity or gas," she said.

And in Mykolaiv in the south, Russian shelling killed at least one person and injured several others, Ukrainian military officials said late Thursday.

- Financial squeeze -

Led by the United States, Western nations have pumped arms and military supplies into Ukraine to help it survive the onslaught.

Bridget Brink, the new US ambassador to Kyiv, promised Thursday that the United States would "help Ukraine prevail against Russian aggression," after presenting her credentials to Zelensky.

Earlier this week, the United States announced that it was sending more advanced Himar multiple rocket launch systems to Ukraine.

The mobile units can simultaneously fire multiple precision-guided munitions at targets up to 80 kilometres away.

They are the centrepiece of a $700 million package that also includes air-surveillance radar, ammunition, helicopters and vehicles.

Kremlin spokesman Dmitry Peskov accused Washington of "adding fuel to the fire," although US officials insist Ukraine has promised not to use them to strike inside Russia.

Beyond sending arms to Ukraine, Western allies have also sought to choke off Russia's financial lifeline in a bid to get Putin to change course.

Ramping up an already long list of embargoes, the United States blacklisted Putin's money manager and a Monaco company that provides luxury yachts to Moscow's elite.

Across the Atlantic, EU nations agreed new sanctions that would halt 90 percent of Russian oil imports to the bloc by the end of the year.

- Oil move disappoints -

Russia warned that European consumers would be the first to pay the price for the partial oil embargo.

Major crude producers agreed to boost output by about 50 percent more a month in an effort to calm an overheated market and ease pressure on inflation.

But the move disappointed investors, and prices rose following the announcement.

The war risks triggering a global food crisis, as Ukraine is one of the world's top grain producers.

It was already translating into higher costs for essentials from cereals to sunflower oil to maize, with the poorest among the hardest hit.

The head of the African Union, Senegalese President Macky Sall, is to visit Russia on Friday for talks with Putin.

The visit is aimed at "freeing up stocks of cereals and fertilisers, the blockage of which particularly affects African countries", along with easing the Ukraine conflict, Sall's office said.

© Agence France-Presse