Russian troops raised their flag on a mine ventilation shaft on the outskirts of the town, which had a population of more than 10,000 people before the war, RIA said, citing an unidentified source in the Russian military.
"Ukrainsk is ours," said Yuri Podolyaka, an Ukrainian-born pro-Russian military blogger, adding that Russian forces had taken the city "almost intact" allowing them to use it as a base for further offensive operations.
There was no immediate comment from the Russian or Ukrainian defence ministries.
Media reports say the number of Ukrainians and Russians killed or wounded in the war is one million. (AP PHOTO)
Reuters was unable to immediately verify battlefield claims from either side due to reporting restrictions in the war zone.
Russian forces had encircled Ukrainsk earlier this month as they advanced westwards towards Pokrovsk, part of what Russian President Vladimir Putin says is a primary goal to take all of the Donbas region.
Podolyaka said that Hirnyk, a town to the south with a pre-war population of about 10,000, and Selydove, a town to the north with a pre-war population of more than 20,000, were the next targets.
Since Russia sent its army into Ukraine in February 2022, the war has largely been a story of grinding artillery and drone strikes along a heavily fortified 1000km front involving hundreds of thousands of soldiers.
Russia in August advanced at its fastest monthly pace in two years, according to open source maps, although Ukraine also took a chunk of Russia's Kursk region in a surprise August 6 incursion.
Russian forces, which have taken about a fifth of Ukraine, control 98.5 per cent of the Luhansk region and 60 per cent of the Donetsk region, according to the same sources.
Together, the two regions make up the Donbas, which is the cradle of the war.
After a pro-Russian president was toppled in Ukraine's 2014 Maidan Revolution, Russia annexed Crimea and pro-Russian protests broke out in parts of the Donbas, where Russia began supporting separatist forces.
Russia said on Tuesday it had repelled five new attempts by Ukrainian forces to smash through its border into the Kursk region, bringing the total number of reported attacks on the border to 26 in just the past six days.
The number of Ukrainians and Russians killed or wounded in the war has reached roughly one million, the Wall Street Journal reported.
