Russia said it had decided to withdraw from the outcrop as a “gesture of goodwill” to show it was not obstructing United Nations efforts to open a humanitarian corridor allowing grains to be shipped from Ukraine.
Ukraine said it had driven the Russian forces out after an 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.”
In another boost for Ukraine’s struggle to beat back the Russian invasion, the United States said it would provide another $US800 million ($A1.2 billion) in weapons and military aid.
US President Joe Biden, speaking after a NATO summit in Madrid, said the members of the military alliance were united in standing up to Russian President Vladimir Putin.
“I don’t know how it’s going to end but it will not end with Russia defeating Ukraine,” Biden told a news conference.
“We are going to support Ukraine for as long as it takes.”
The retaking of Snake Island came after weeks in which momentum in the four-month-old 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.
The Ukrainian military posted an image on Facebook of what appeared to be the island, seen from the air, with several 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,” it said.
Ukrainian Brigadier General Oleksii Hromov said Ukrainian forces were not yet occupying the island but would do so.
The 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.
“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,” Rob Lee of the US-based Foreign Policy Research Institute, said.
US Secretary of State Antony Blinken has accused Russia of deliberately causing world hunger as “blackmail”.
Russia denies blocking the ports and blames food shortages on foreign 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,” Putin said on Thursday.
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.
Russia had defended the island since February despite Ukraine claiming to inflict severe damage, sinking supply vessels and destroying Russian fortifications.
Mathieu Boulegue of the Chatham House think tank in London cautioned that the Russian move could free up the assets deployed on Snake Island to strengthen its forces elsewhere on the Black Sea coast.
“We should not be fooled by it… It might be short-term relief but there will be long-term pain,” he said.
In the battle for Donbas, Ukrainian authorities said they were trying to allow remaining residents leave the city of Lysychansk, where they believe about 15,000 people remain.
Russian forces have been trying to encircle Lysychansk since they captured Sievierodonetsk, on the opposite side of the Siverskyi Donets River, last week after weeks of heavy fighting.
“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.
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