(Reuters) - Russian forces have seized control of about a third of the Ukrainian city of Sievierodonetsk, but their assault was taking longer than they had hoped, according to a Moscow-backed separatist leader quoted in a TASS news agency report.

       Russian shelling has reduced much of Sievierodonetsk to ruins and Russian troops have entered the city's southeastern and northeastern fringes, but the Ukrainian defence has slowed the wider Russian campaign across the Donbas region.

       "We can say already that a third of Sievierodonetsk is already under our control," TASS quoted Leonid Pasechnik, the leader of the Luhansk People's Republic, as saying in a report on Tuesday morning.

       Pasechnik told the Russian state news agency that fighting was raging in the city, but Russian forces were not advancing as rapidly as might have been hoped.

       "But we want, above all, to maintain the city's infrastructure," he said.

       The advance of Russian troops was complicated by the presence of several large chemical plants in the Sievierodonetsk area, TASS reported.

       Luhansk was recognised as independent by Russia when Moscow launched its invasion of Ukraine on Feb. 24., although Kyiv and its Western allies consider it part of Ukraine. Russia has been pressing to seize the entire Donbas region, consisting of the Luhansk and Donetsk regions which Moscow claims on behalf of separatist proxies.

       (Reporting in Winnipeg by Ronald Popeski and in Melbourne by Lidia Kelly; Writing by Lidia Kelly; Editing by Simon Cameron-Moore)