DNIPRO, Ukraine—Russia is regrouping its forces in eastern Ukraine and launching offensives along five lines of attack, Ukraine’s armed forces said on Saturday as officials in Kyiv and Western capitals continue to warn of a major Russian push to gain territory. The main focus of Russia’s offensive remains the besieged city of Bakhmut in the Donetsk region, which it is seeking to surround and capture. The Ukrainian military said it had repelled multiple attacks, inflicting significant losses for the Russians.