KYIV, June 5 (Reuters) - Kyiv was rocked by several explosions early on Sunday, the mayor of the Ukrainian capital said, a day after officials said its troops had recaptured a swath of the eastern battlefield city of Sievierodonetsk in a counter-offensive against Russia.

"Several explosions in Darnytskyi and Dniprovskyi districts of the capital," Kyiv Mayor Vitali Klitschko wrote on the Telegram messaging app. "Services are already working on site."

A Reuters witness saw smoke in the city after the explosions.

At least one person was hospitalised but no deaths had been reported as of early Sunday, Klitschko said. Other officials said the Russian bombardment appeared to be targeting the railway network.

Despite continuing Russian attacks on Ukraine and the widespread destruction, Kyiv has been relatively calm in recent weeks after Moscow turned its military focus to the east and south, especially an intense battle for Sievierodonetsk.

Russia has concentrated its forces on the factory city for one of the biggest ground battles of the war, with Moscow appearing to bet its campaign on capturing one of the two eastern provinces it claims on behalf of separatist proxies.

Serhiy Gaidai, the governor of the Luhansk region that includes Sievierodonetsk, said on Sunday Ukrainian forces controlled about half the city after recapturing a large chunk from Russian troops.

"It had been a difficult situation, the Russians controlled 70% of the city, but over the past two days they have been pushed back," Gaidai said on Ukraine's television. "The city is now, more or less, divided in half."

The claims could not be independently verified.

Britain's defence ministry said on Sunday that Ukrainian counter-attacks there over the past 24 hours were likely to blunt any operational momentum that Russian forces had previously gained.

Moscow said its own forces were making gains in the city. Ukraine's army said Russian forces continued to mount assault operations with the aid of artillery and controlled the eastern part of Sievierodonetsk.

"The situation is tense, complicated," Mayor Oleksandr Stryuk told national television on Saturday, saying there was a shortage of food, fuel and medicine. "Our military is doing everything it can to drive the enemy out of the city."

Both sides claim to have inflicted huge casualties in the fighting, a battle that military experts say could determine which side has the momentum for a prolonged war of attrition in coming months.

Ukrainian officials said at least eight people were killed and 11 injured in Russian shelling in the neighbouring Donetsk region. Donetsk and Luhansk make up the wider Donbas region where Russia hopes to take control.