SIEVIERODONETSK, Ukraine (Reuters) - Ukraine said it had recaptured a chunk of the factory city of Sievierodonetsk, the focus of a Russian offensive to take the eastern Donbas region, and could hold it for up to two weeks as fighting raged on Saturday.

Sergiy Gaidai, governor of Luhansk province, told national television on Friday that Ukrainian troops had retaken 20% of the territory they had lost in Sievierodonetsk.

It was "not realistic" that the city would fall in the next two weeks even though Russian reinforcements were being deployed, he said.

"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," said Gaidai.

His claim of Ukrainian advances could not immediately be verified. Reuters reached Sievierodonetsk on Thursday and was able to verify that Ukrainians still held part of the city.

Ukraine's military said on Saturday Russia had reinforced its troops and had used artillery to conduct "assault operations" in the city. But it said Russian forces had retreated after failed attempts to advance in the nearby town of Bakhmut and cut off access to Sievierodonetsk.

Gaidai said in a social media post that four people were killed in Russian attacks in the region on Friday, including a mother and a child.

In Ukraine's southern Odesa region on Saturday morning, a missile hit an agricultural storage unit, wounding two people, the regional administration's spokesman wrote on Telegram.

Two people died and at least two were injured in Russian shelling of civilian infrastructure in the northeastern Kharkiv region on Friday, Ukraine’s Interfax reported, citing emergency services.

The war in Ukraine marked its 100th day on Friday. Tens of thousands have died, millions have been uprooted from their homes and the global economy disrupted since Moscow’s forces were driven back from Kyiv in the first weeks of the conflict.

Russian President Vladimir Putin denied on Friday that Moscow was preventing Ukrainian ports from exporting grain, blaming rising global food prices on the West.

"We are now seeing attempts to shift the responsibility for what is happening on the world food market, the emerging problems in this market, onto Russia," he said on national television.

He said the best solution would be for Western sanctions on Russia's ally Belarus to be lifted and for Ukraine to export grain through that country.

Ukrainian officials are counting on advanced missile systems that the United States and Britain recently pledged to swing the war in their favour, and Ukrainian troops have already begun training on them.

While Ukraine's resistance has forced Putin to narrow his immediate goal to conquering the entire Donbas region, Ukrainian officials said he remains intent on subduing the whole country.

"Putin's main goal is the destruction of Ukraine. He is not backing down from his goals, despite the fact that Ukraine won the first stage of this full-scale war," Ukrainian Deputy Defence Minister Hanna Malyar told national television on Friday.

A Russian government spokesman said "certain results have been achieved" in the war and Moscow will continue its military operations until all goals are met.

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

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

In neighbouring Donetsk province, Russian troops were just 15 km (9 miles) 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.

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.

Russia controls around a fifth of the country, about half of that seized in 2014 and the rest 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 to 100 soldiers every day.

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

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

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, with more than 20 million tonnes of grain stuck in silos.

United Nations aid chief Martin Griffiths on Friday ended two days of "frank and constructive discussions" with Russian officials in Moscow on facilitating exports of Ukraine grain from Black Sea ports, a U.N. spokesman said.

The talks came as U.N. Secretary-General Antonio Guterres tries to broker what he calls a "package deal" to resume both Ukrainian food exports and Russian food and fertiliser exports.

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

(This story corrects seventh paragraph to say four people were killed in Luhansk on Friday, not Saturday.)