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Kyiv has said Moscow now controls 20 per cent of Ukrainian territory on the eve of the war's 100th day as Russian forces hammered Ukrainian positions in the Donbas region on Thursday.
Russian troops have focused on capturing eastern Ukraine since being repelled from around the capital Kyiv.
Although their advance has been much slower than Moscow expected, Russian forces have expanded control beyond the 43,000 square kilometres taken when Russia seized Crimea and parts of the Donbas in 2014.
“Today, about 20 per cent of our territory is under the control of the occupiers,” Ukrainian President Volodymyr Zelenskyy said in an address to politicians in Luxembourg.
Thousands of people have been killed and millions forced to flee, with Ukraine's east now bearing the brunt of Russia's assault, which Mr Zelenskyy said was killing up to 100 Ukrainian soldiers every day.
Western nations have pumped arms and military supplies into Ukraine to help it survive the onslaught.
But Ukraine's allies need to brace for a gruelling “war of attrition”, Nato chief Jens Stoltenberg said on Thursday after talks with US President Joe Biden in Washington.
“We just have to be prepared for the long haul,” Mr Stoltenberg said, while reiterating that Nato does not want direct confrontation with Russia.
On the ground, street battles were raging in the industrial centre of Severodonetsk in Luhansk, part of the Donbas.
The city is a vital target for Moscow, which already controls 80 per cent of the area, but Luhansk regional governor Sergiy Gaiday vowed Ukrainian forces would fight “until the end”.
Severodonetsk's Azot factory, one of Europe's biggest chemical plants, was attacked by Russian soldiers who fired on one of its administrative buildings and a warehouse where methanol was stored.
Ukrainian troops are still holding an industrial zone, Mr Gaiday said, a situation reminiscent of Mariupol, where a huge steelworks became the south-eastern port city's last holdout until they surrendered late last month.
US and Nato: 'Putin wanted less Nato, he got more troops and members'
In the city of Sloviansk, about 80 kilometres from Severodonetsk, residents spoke of constant bombardments by Russian troops.
Paramedic Ekaterina Perednenko said she had only returned to the city five days ago but soon realised that she would have to leave again.
“It's very difficult here," she said. "Shooting is everywhere, it's scary. No water, electricity or gas."
Another local, Leonid, said he was also leaving the city and would seek refuge elsewhere in Europe.
“I feel pain” he said. "The most prominent feeling I have is that we didn't deserve this. We don't understand why we are punished like this."
Valeriy Zaluzhnyi, Commander in Chief of Ukraine's armed forces, pleaded for more modern arms from Nato and said “the enemy has a decisive advantage in artillery”.
“It will save the lives of our people,” he said.
Bridget Brink, the new US ambassador to Kyiv, promised on Thursday that the US would “help Ukraine prevail against Russian aggression” after presenting her credentials to Mr Zelenskyy.
This week, the US announced that it was sending more advanced, Himars multiple rocket launch systems to Ukraine.
The mobile units can simultaneously fire several precision-guided munitions up to 80km.
They are the centrepiece of a $700 million military assistance package that includes air-surveillance radar, more Javelin short-range anti-tank missiles, artillery ammunition, helicopters, armoured vehicles and spare parts.
Kremlin spokesman Dmitry Peskov accused Washington of “adding fuel to the fire”, although US officials insist Ukraine has promised not to use them to carry out strikes inside Russia.
Zelenskyy says situation in Donbas is 'very, very difficult'
Beyond sending arms to Ukraine, western allies have also sought to choke off Russia's financial lifeline in a bid to get Russian President Vladimir Putin to change course.
Stepping up an already long list of embargoes, the US blacklisted Mr Putin's money manager and a Monaco company that provides luxury yachts to some of Moscow's elite.
Across the Atlantic, EU nations agreed on new sanctions that would halt 90 per cent of Russian oil imports to the bloc by the end of the year.
Russia said European consumers would be the first to pay the price for the partial oil embargo.
But some relief was in view for the overheated oil market as producers including Saudi Arabia agreed to add 648,000 barrels per day to the market in July, up from 432,000.
The war has wrecked Ukraine's economy, forcing the central bank to more than double its key interest rate on Thursday to prop up the hryvna, the local currency.
Zelenskyy says 200,000 children among Ukrainians forcibly taken to Russia
But the war carries far wider consequences, too, with risks that it could trigger a global food crisis.
Ukraine — one of the world's main producers — will likely export only half the amount of grain that it did in the previous season, the Ukrainian Grain Association said.
The conflict was already translating into higher costs for essentials, from cereals to sunflower oil to corn, with the poor among the hardest hit.
The head of the African Union, Senegalese President Macky Sall, is to visit Russia on Friday for talks with Mr Putin.
The trip is aimed at “freeing up stocks of cereals and fertilisers, the blockage of which particularly affects African countries”, along with easing the Ukraine conflict, Mr Sall's office said.
