Russian forces fired on Ukrainian positions and residential areas in the country’s east overnight and unleashed new rocket attacks, while fortifying positions there with new deployments of air-defense systems, Ukrainian officials said Tuesday.

       Ukrainian authorities are still investigating allegations by Ukrainian forces that they came under a Russian chemical attack in Mariupol, where Russia is still fighting to consolidate gains in the city, which Moscow sees as crucial to its efforts in taking over eastern Ukraine and pushing westward. The Russian government hasn’t commented publicly about the alleged use of chemical weapons in Mariupol.

       Russia’s Defense Ministry said it was launching air and artillery strikes on Ukrainian positions in Mariupol.

       In his first extended comments on the war since last month, Russian President Vladimir Putin said Tuesday that the main goal of the offensive was “to help people in Donbas," in another sign that his forces would launch an assault on the eastern region. He added that “there is no doubt" the goal will be achieved, speaking during a visit to the Vostochny Cosmodrome, a Russian spaceport in the country’s Far East, along with his Belarusian counterpart, Alexander Lukashenko.

       The U.K. Defense Ministry said fighting was continuing in Ukraine’s south near the cities of Kherson and Mykolayiv and that Russia is continuing to emphasize Donbas in a shifting war effort. British intelligence said Russia would continue to redeploy its troops from Belarus to the Ukrainian east and likely try to take the eastern city of Kramatorsk, the site of a deadly missile attack last week.

       Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov said Tuesday that high-precision air- and sea-based missiles overnight destroyed an ammunition depot and a hangar with Ukrainian aircraft at the Starokostiantyniv military airfield in the western Khmelnytskyi region, as well as an ammunition depot near Havrylivka in the Kyiv region.

       Ukraine’s General Staff said Russians were boosting air-defense capabilities near Melitopol and Ilovaisk and added that it had repulsed six enemy attacks in the Donetsk and Luhansk regions, destroying four Russian tanks and eight artillery systems. The Ukrainian Air Force said it had downed a Russian plane, two helicopters and four drones.

       Ukrainian authorities said they are investigating a claim by their troops in the port city of Mariupol that they came under a Russian chemical weapons attack Monday.

       The Ukrainian unit deployed in Mariupol, the Azov regiment, said Russian forces dropped an unknown chemical substance from a drone, causing respiratory and nervous-system symptoms among its troops and civilians. No independent evidence of the attack has emerged from Mariupol.

       Railway connections were disrupted overnight in the Russian town of Shebekino in the Belgorod region, near the Ukrainian border northeast of Kharkiv, the regional governor told Russia’s RIA news agency. There were no casualties, he said, and the cause of the disruption was being investigated. Russia’s Investigative Committee said it is looking into what caused the disruption to the railway line.

       Russia heavily uses the railways to reinforce and resupply its troops preparing the Donbas offensive, and Shebekino sits on one of its main rail connections to the area. Russia last month said two Ukrainian helicopters launched an airstrike on a fuel depot in Belgorod, a claim Kyiv didn’t confirm or deny.

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       —Mauro Orru and Evan Gershkovich contributed to this article.

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