Ukraine on Tuesday said it had carried out a successful long-range rocket strike against Russian forces in southern Ukraine, territory it says it is planning to retake in a counter-offensive using hundreds of thousands of troops.

According to Ukraine, the strike hit an ammunition dump in the town of Nova Kakhovka, about 35 miles (55 kilometers) east of the important Black Sea port city of Kherson, and killed 52 Russians. It came after Washington supplied Ukraine with HIMARS mobile artillery systems, which Kyiv says its forces are starting to use with ever greater efficacy.

Russia’s Tass news agency offered a different account, saying that the target was a mineral fertilizer storage facility that exploded, and that a market, hospital and houses were damaged. Some of the ingredients in fertilizer can be used for ammunition.

Ukrainian authorities also said that Russia struck the southern city of Mykolaiv on Tuesday morning, hitting two medical facilities and residential buildings. Four people were wounded in the shelling attack, Mykolaiv regional governor Vitaliy Kim said on Telegram.

Air raid sirens sounded early Tuesday morning in the western city of Lviv and other areas of Ukraine as Russian forces continued to make advances.

According to a Tuesday intelligence briefing from the British military, Russia is continuing to make "small, incremental gains" in Ukraine’s eastern Donetsk region, where heavy fighting led the province's governor last week to urge its 350,000 remaining residents to move to safer places in western Ukraine.

Ukraine's deputy prime minister on Sunday urged civilians in Kherson to urgently evacuate as Ukraine's armed forces were preparing a counter-attack there.

"It's clear there will be fighting, there will be artillery shelling ... and we, therefore, urge (people) to evacuate urgently," Deputy Prime Minister Iryna Vereshchuk said on national television.

Yet many in Donbass, a fertile industrial region in eastern Ukraine made of the Donetsk and Luhansk provinces, refuse – or are unable – to flee, despite scores of civilians being killed and wounded each week.

The death toll in a Russian rocket attack that struck an apartment building in eastern Ukraine on Saturday has risen to 34. The head of the Donetsk regional military administration, Pavlo Kyrylenko, made the announcement on social media, saying nine wounded people had been recovered from the building in Chasiv Yar.