Ukraine has retaken more than 1,000 sq km (390 sq miles) of territory and over 20 villages, the president, Volodymyr Zelenskiy, has announced, as its troops wage a counteroffensive in the south and the east of the country. Zelenskiy also released a video late on Thursday in which Ukrainian soldiers said they had taken the key eastern town of Balakliia, which has a population of 27,000 people. Earlier, Brig Gen Oleksiy Gromov said at a briefing that Ukrainian armed forces had advanced up to 50km (31 miles) into Russian lines and that “the total amount of territory returned to Ukrainian control in the Kharkiv and Pivdennyi Buh directions stands at more than 700 sq km”. Gromov said Ukrainian troops had advanced up to 3km on the Sloviansk front in the east and recaptured a settlement called Ozerne. This is the first time Kyiv has disclosed details of its recent counteroffensive since last week so as not to compromise the operation. The general singled out the role of Turkish-made Bayraktar drones. “Enemy infantry and motorised artillery units unprotected by air defence systems become easy prey for our Bayraktars, the quantity of which is always increasing, thanks to our volunteers,” he said. A top US general, Mark Milley, described the progress of Ukraine’s counteroffensive as “steady” and “deliberate”, and pointed in particular to the impact of US-supplied high-mobility artillery rocket systems (Himars) in supporting Ukraine’s advance. “We are seeing real and measurable gains from Ukraine in the use of these systems,” Gen Milley, the chair of the joint chiefs of staff, said in Ramstein, Germany. “For example, Ukrainians have struck over 400 targets with the Himars and they’ve had a devastating effect. “Russian lines of communication and supply channels are severely strained. It is having a direct impact on the Russian ability to project and sustain combat power. Russian command and control in their headquarters have been disrupted and they’re having great difficulty in supplying their forces and replacing their combat losses.” Milley and the US defence secretary, Lloyd Austin, were in Ramstein to meet their counterparts from about 50 countries to coordinate military support for Ukraine on a day when the US secretary of state, Antony Blinken, announced another $2.8bn in arms supplies, both short- and long-term, for Ukraine and neighbouring states. A state department official, speaking on condition of anonymity, said the US package would “bolster the security of Ukraine and 18 of its neighbours, including … many of our Nato allies as well as other regional security partners”. Blinken made the arms announcement on a surprise visit to Kyiv. His arrival, which was not publicly expected until he landed, came after Zelenskiy, reported “good news” from the war’s frontlines. In a Wednesday evening address, Zelenskiy cited “the extremely successful hits in areas where the occupiers are concentrated”, and thanked Ukrainian artillery troops for what he said were successful strikes against Moscow’s forces in the south. Also on Wednesday, an official representing the Russian-controlled self-proclaimed republic in Donetsk confirmed that Ukraine had launched a surprise counterattack in the north-east Kharkiv region and “encircled” Balakliia, an eastern town of 27,000 people situated between Kharkiv and Russian-occupied Izium. Unverified footage circulating on social media showed what looked like a Ukrainian soldier posing in front of an entrance sign for Balakliia. The US Institute for the Study of War (ISW), which follows the fighting in detail, said the Ukraine counterattack in the east drove Russian forces back to the north side of the Siverskyi Donets and Serednya Balakliika rivers and that Kyiv had retaken 400 sq km of territory in the east of the Kharkiv region. It appeared that Ukraine forces had also recaptured Verbivka and that Russian forces may have destroyed bridges to prevent Ukrainian fighters from pursuing them, ISW said. “Russia’s deployment of forces from Kharkiv and eastern Ukraine to Ukraine’s south is likely enabling Ukrainian counterattacks of opportunity,” it said. The long-awaited counteroffensive came at a crucial moment in the conflict. After months in which Ukraine’s fate seemed sealed, with Moscow cornering Ukraine in the Donbas and threatening to advance towards Odesa, the reconquest of territory by the Ukrainian armed forces seems to have raised the morale of the people, who are resigned to a conflict that could last for years. “Each success of our military in one direction or another changes the general situation along the entire frontline in favour of Ukraine,” Zelenskiy said. “The more difficult it is for the occupiers, the more losses they have, the better the positions of our defenders in Donbas will be.” The Ukrainian counteroffensive in the east does not seem to have stopped the Russian bombing in the city of Kharkiv. On Thursday, the regional governor of Kharkiv, Oleh Synehubov, said two people were killed and five were injured in Russian shelling in the city’s industrial district. Ukraine’s commander-in-chief of the armed forces, Gen Valeriy Zaluzhnyi, gave a detailed assessment of the war to date in rare public comments published on Wednesday and warned of the threat of Russia using nuclear weapons in Ukraine, which would create the risk of a “limited” nuclear conflict with other powers, according to an opinion piece attributed in his name in Ukraine’s state news agency Ukrinform. Zaluzhnyi said the “direct threat” of Russia’s possible use of tactical nuclear weapons had had a major influence on the adoption of relevant decisions. “Another factor is the direct threat of the use by Russia, under certain circumstances, of tactical nuclear weapons,” he said. “Battles on the territory of Ukraine have already demonstrated how much the Russian Federation neglects the issues of global nuclear security even in a conventional war. It is hard to imagine that even nuclear strikes will allow Russia to break Ukraine’s will to resist. But the threat that will emerge for the whole of Europe cannot be ignored.” Heavy fighting was raging on Thursday in areas near the Russian-occupied Zaporizhzhia nuclear power station in Ukraine after Kyiv said it may have to shut down the plant to avoid disaster. The general staff of Ukraine’s armed forces said in its daily update that some villages and communities near the plant were heavily shelled in the 24 hours to Thursday morning from “tanks, mortars, barrel and jet artillery”. The occupation of the nuclear power plant has prompted fears of a nuclear disaster as both sides trade blame for shelling the site. On Wednesday, the head of the IAEA, Rafael Grossi, who was in Rome for a meeting with the Italian prime minister, Mario Draghi, told La Repubblica newspaper that he was moved when he visited the plant. When asked what he thought about Vladimir Putin and Zelenskiy, who accused him of not saying who was to blame for the strikes on the plant, Grossi replied: “Being a judge, the referee between two contenders, is not my mandate. Indeed, if it was, it would cancel my utility as guarantor of the safety of the nuclear power plant.”