Ukrainian President Volodymyr Zelensky vowed victory on the 100th day of Russia's invasion on Friday, even as Russian troops pounded the eastern Donbas region.

Thousands of people have been killed, millions sent fleeing and towns turned into rubble, since Russian President Vladimir Putin ordered his troops into Ukraine on February 24.

Russia's advance has been slowed by a fierce Ukrainian resistance which repelled them from around the capital and forced Moscow to shift its aims towards capturing the east.

"Victory will be ours," Zelensky said in a video address similar to one he posted at the onset of the war outside government buildings in Kyiv.

But Kremlin spokesman Dmitry Peskov said: "certain results have been achieved", pointing to the "liberation" of some areas from what he called the "pro-Nazi armed forces of Ukraine".

The West has sent ever more potent weapons to Ukraine and piled on ever more stringent sanctions, with the EU also on Friday formally adopting a ban on most Russian oil imports.

Putin's alleged girlfriend, former gymnast Alina Kabaeva, was also added to an assets freeze and visa-ban blacklist, along with Russian army personnel suspected of war crimes.

At the same time, the United Nations said it was leading intense negotiations with Russia to allow tens of millions of tons of grain to leave Ukrainian ports to avert a global food crisis.

Food crisis

"I am optimistic that something could give in, something could be made," said Amin Awad, the UN crisis coordinator for Ukraine, voicing hope that we could "see a breakthrough".

Russian troops now occupy a fifth of Ukraine's territory and Moscow has imposed a blockade on the country's Black Sea ports.

The UN has warned that especially African countries, which imported more than half of their wheat consumption from Ukraine and Russia, face an "unprecedented" crisis caused by the conflict. 

Food prices in Africa have already exceeded those in the aftermath of the 2011 Arab springs and the 2008 food riots.

On Friday, Putin met the head of the African Union, Senegalese President Macky Sall, at his Black Sea residence in Sochi.

At the opening of those talks, Sall told Putin to "become aware" African countries "are victims" in the Ukraine conflict.

After the talks with Putin, Sall said he was "very reassured" and that the Russian leader was "committed and aware that the crisis and sanctions create serious problems for weak economies".

Putin's troops are now concentrating their forces in the Donbas, in the east, where some of the fiercest fighting is centred on the industrial hub city of Severodonetsk.

Journalists wounded

Fighting continues in Severodonetsk's city centre, the president's office said, adding that the invaders were "shelling civilian infrastructure and Ukrainian military".

"For 100 days, they have been levelling everything", Lugansk regional governor Sergiy Gaiday said on Telegram.

Two journalists working for the international news agency Reuters were lightly wounded and their driver killed on Friday while on their way to Severodonetsk, a company spokesperson said.

The agency said the group was travelling "in a vehicle provided by the Russian-backed separatists and driven by an individual assigned by the separatists".

The French foreign ministry also on Friday said a French volunteer fighter in Ukraine had been killed in combat, following reports that the man died in artillery fire in the neighbouring Kharkiv region.

Meanwhile in areas around the capital Kyiv which Russian troops retreated from at the end of March, some residents are still in desperate need.

At an aid distribution point in Horenka, northwest of Kyiv, a tearful Hanna Viniychuk, 67, said she had come for some basic necessities after losing her home in Russian bombardment.

"I'm grateful for this help," she said.

'Nothing to come back to'

Ukrainian troops were still holding an industrial zone, Gaiday said, a situation reminiscent of Mariupol, where a steelworks was the south-eastern 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 per cent of infrastructure and housing had been destroyed, while the 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, the mayor has urged residents to evacuate as bombing intensified and water and electricity are cut off.

Student Gulnara Evgaripova, 18, recounted heavy bombardments as she boarded a minibus to leave the city.

"The situation is getting worse, the explosions are stronger and stronger, and the bombs are falling more often," she told AFP.

Ekaterina Perednenko, a paramedic, said: "I am scared that there will be nothing to come back to".