SIEVIERODONETSK, UKRAINE (REUTERS) - Ukraine on Friday (June 3) said it recaptured a large chunk of territory in fierce fighting for Sievierodonetsk and foiled an attempt by Russian troops to advance from the devastated eastern industrial city on the 100th day of Moscow's invasion.

Ukraine's defence minister said his soldiers were already training in Europe to operate new, advanced missile systems pledged this week by the United States and Britain, which Kyiv hopes will help swing the battle in its favour.

A war that Western countries believe Russia planned to win within hours has ground on for more than three months at a cost of thousands of lives and disruption to the global economy.

Moscow was driven back from Kyiv and launched a huge new assault in the east.

Rejecting Western criticism that the war is to blame for rising global food prices hurting poor countries, Russian President Vladimir Putin denied Moscow was preventing Ukrainian ports from exporting cereals.

Reuters reached Sievierodonetsk on Thursday and was able to verify that Ukrainians still held part of the city. Troops drove towards a plume of black smoke at high speed over roads littered with wrecked armoured vehicles. One soldier sat in the back seat, his face streaked with blood from injuries.

At another location in the city, Ukrainian troops, including foreign volunteers, unloaded weapons from a truck.

"We're gonna push the Russians back. It will take a day, a month, or a year it does not f***ing matter. We are on the right side of history," said Zurab Kakalidze, a Georgian who described himself as "just a 22-year-old kid."

On Friday, the Ukrainian head of the Luhansk region told national television that Ukrainian troops had recaptured around 20 per cent of the territory they had lost to Russian forces in Sievierodonetsk.

"Whereas before the situation was difficult, the percentage (held by Russia) was somewhere around 70 per cent, now we have already pushed them back by approximately 20 per cent," Serhiy Gaidai said.

Reuters could not independently verify Gaidai's claim.

Two Reuters journalists were injured and a driver killed after their vehicle came under fire as they tried to reach Sievierodonetsk from an area controlled by Russian-backed separatists.

The past weeks have seen Russia pour forces into the battle for the city known for its large chemicals factory. Russia must capture it to achieve its stated aim of holding all of Luhansk province.

Both sides have been taking punishing losses there in a street-by-street battle that could set the trajectory for a long war of attrition.

Gaidai said the Russians were shelling Ukrainian positions for hours and then advancing, only to be repulsed by defenders who had not been hurt, before repeating the pattern.

"This is how they are moving forward, step by step, because with artillery, aircraft, mortars, they are simply destroying everything," he said.

"But as soon as we have enough Western long-range weapons, we will push their artillery away from our positions. And then, believe me, the Russian infantry, they will just run."

Russian soldiers attempted to advance towards Lysychansk, across the Siverskyi Donetsk River from Sievierodonetsk but were forced to retreat, Ukraine's military general staff said.

In neighbouring Donetsk province, also a target of Moscow's eastern offensive, Russian troops were just 15km outside the city of Sloviansk, regional governor Pavlo Kyrylenko told Reuters.

Donetsk will not fall quickly, but needs more weapons to keep the attackers at bay, Kyrylenko said.

Washington said this week it expected Ukrainians would need three weeks of training to use the rockets, whose range of up to 80km could help negate Russia's artillery fire-power advantage.

US President Joe Biden told reporters a negotiated settlement in Ukraine would be needed at some point, but in the meantime the US would help Ukrainians defend themselves.

Ukraine's President Volodymyr Zelensky, said in an overnight address that Kyiv was expecting more "good news" on foreign arms, after the latest US$700 million US weapons package for Ukraine.

"Victory will be ours," he said later on Friday, in a video address from outside his Kyiv office to mark 100 days of the war.

Slow but steady progress

Moscow says the Western weapons will pour "fuel on the fire," but will not change the course of what it calls a"special military operation" to disarm Ukraine and rid it of dangerous nationalists.

Despite being driven from the north of Ukraine in March after a failed assault on the capital, Russia still controls around a fifth of the country, about half seized in 2014 and half captured since launching its invasion on Feb 24.

For both sides, the massive Russian assault in the east in recent weeks has been one of the deadliest phases of the war, with Ukraine saying it is losing 60-100 soldiers every day.

Moscow has made slow but steady progress, squeezing Ukrainian forces inside a pocket in Luhansk and Donetsk provinces, but so far failing to encircle them.

Kyiv, meanwhile, hopes the Russian advance will drain Moscow's forces enough for Ukraine to recapture territory in months to come.

Resnikov said he hoped it was "absolutely realistic" to drive Russians out of eastern Ukraine this year, but he did not hold "tarot cards" to tell exactly when.

Famine, destabilisation

The war has had a devastating impact on the global economy, especially for poor food-importing countries. Ukraine is one of the world's leading sources of grain and cooking oil, but those supplies were cut off by the closure of its Black Sea ports.

"Failure to open those ports will result in famine," UN crisis coordinator Amin Awad said in Geneva, saying a grain shortage could affect 1.4 billion people and trigger mass migration.

Kyiv and its allies blame Moscow for blockading the ports, which Ukraine has mined to prevent a Russian amphibious assault.

Putin blamed Western sanctions and said sanctions should be lifted on Belarus, to allow Ukraine to export through its neighbour, an ally of Moscow.