Russian forces abandoned the strategic Black Sea outpost of Snake Island on Thursday (local time), in a major victory for Ukraine that could loosen the grip of Russia's blockade on Ukrainian ports.

Russia's defence ministry said it had decided to withdraw from the outcrop as a "gesture of goodwill" to show Moscow was not obstructing UN efforts to open a humanitarian corridor allowing grains to be shipped from Ukraine.

Ukraine said it had driven the Russian forces out after a massive artillery and missile assault overnight.

"KABOOM!" tweeted Andriy Yermak, Ukrainian President Volodymyr Zelenskiy's chief of staff. "No Russian troops on the Snake Island anymore. Our Armed Forces did a great job."

Ukraine's southern military command posted an image on Facebook of what appeared to be the island, seen from the air, with at least five huge columns of black smoke rising above it.

"The enemy hurriedly evacuated the remains of the garrison with two speed boats and probably left the island. Currently, Snake island is consumed by fire, explosions are bursting."

Oleksii Hromov, brigadier general in Ukraine's armed forces, told a briefing that Ukrainian forces were not yet occupying the island, "but they will be, believe me".

'BIG VICTORY'

The bare rocky outcrop overlooks sea lanes to Odesa, Ukraine's main Black Sea port, where Russia is blocking food cargos from one of the world's leading grain suppliers.

Snake Island has held the world's attention since Russia seized it on the war's first day, when a Ukrainian guard, ordered by Russia's flagship cruiser Moskva to surrender, radioed back "Russian warship: go fuck yourself".

"The most significant aspect is that this could open the door to Ukrainian grain exports from Odesa, which is critical for Ukraine's economy and for the global food supply," tweeted Rob Lee, a senior fellow at the US-based Foreign Policy Research Institute.

Lifting the blockade has been a primary strategic goal of the West: US Secretary of State Antony Blinken has accused Russia of deliberately causing world hunger, as "blackmail."

Moscow denies blocking the ports and blames food shortages on Western sanctions it says limit its own exports.

"We do not prevent the export of Ukrainian grain. The Ukrainian military has mined the approaches to their ports; no one prevents them from clearing those mines and we guarantee the safety of shipping grain out of there," President Vladimir Putin said on Thursday.

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Several military experts said that driving the Russians from Snake Island would not by itself be enough to unblock the ports.

"Does that mean that suddenly the grain flows? No it doesn't really," said Marcus Faulkner, a lecturer of War Studies at King's College London, noting that ports were still mined and that Russia could still intercept cargo ships at sea.

Oleg Zhdanov, a Kyiv-based military analyst, also said the ports could not be opened immediately, but it was still a "big victory in the sense that we are liquidating the Black Sea Fleet's dominance."

Last month Britain's defence ministry said that if Russia were able to consolidate its grip on Snake Island with air defence and coastal defence cruise missiles, it could dominate the northwestern Black Sea. 

Russia had defended the island since February, despite Ukraine increasingly claiming to inflict severe damage, sinking supply vessels and destroying Russian fortifications.

New weapons sent by the West made the Russian garrison even more vulnerable, especially HIMARS, a powerful rocket system supplied by the United States which Ukraine began fielding last week. Lee said Russia's abandonment of the island was "likely a tangible result of NATO arms deliveries to Ukraine".

RUSSIAN MOMENTUM

The Ukrainian victory on Snake Island comes after weeks in which momentum in the conflict appeared to be shifting in favour of Russia, which has focused its firepower on capturing cities and towns in the Donbas region of eastern Ukraine.

It captured the city of Sievierodonetsk last week after weeks of heavy fighting and is now trying to encircle the city of Lysychansk on the opposite side of the Siverskyi Donets River.

Ukrainian authorities said they were trying to evacuate remaining residents from Lysychansk, where they believe around 15,000 people remain.

"Fighting is going on all the time. The Russians are constantly on the offensive. There is no let-up," regional Governor Serhiy Gaidai told Ukrainian television.

An official from the pro-Russian separatist administration in the province told RIA news agency the Lysychansk oil refinery was now fully controlled by Russian and pro-Russian forces, and all roads to Lysychansk were also under their control.

Ukraine says the main road out is largely impassable because of fighting, but the city is not yet fully cut off.

Despite yielding ground and taking punishing losses in the Donbas in recent weeks, Ukraine hopes to inflict enough damage to exhaust Russia's advancing army. Ukrainian forces have been mounting a counter-attack in the south, where Russian-installed proxies have announced preparations for votes to join Russia.

Ukraine's 60th infantry brigade said on Facebook on Thursday that it had retaken the village of Potyomkine in the southern Kherson region. This could not be independently verified.

In Madrid, NATO leaders held the second day of a summit at which the alliance declared Russia was its main adversary and announced plans to put 300,000 troops on a higher alert.

The alliance invited Finland and Sweden to join, and leaders pledged more weapons for Ukraine, including US President Joe Biden who announced the next $800 million tranche of support.

Turkey, which has tried to act as a mediator, said it hoped for progress on an agreement reopening Ukrainian ports. Greece, which boasts the world's largest commercial fleet, signalled it was prepared to send ships to bring out Ukrainian grain.