Ukrainian fighter pilots are being trained to fly F-16 jets, Nato’s secretary general has revealed, as Kyiv claimed further incremental progress in its counteroffensive against Russian forces in the east and south of Ukraine. Nato allies have yet to agree on delivering the so-called fourth-generation US fighters to Ukraine, but Jens Stoltenberg said that the training of Ukrainian personnel was under way. “The fact that training has started provides us with the option to also decide to deliver the planes and then the pilots will be ready to fly them,” the former prime minister of Norway said as he arrived at a meeting of defence ministers in Brussels. The development follows months of internal debate in Washington over the risks of Ukraine deploying F-16s to attack targets on Russian territory and potentially escalating the conflict. The US has control over the jets’ re-export from any country that has them in its arsenal. As recently as February the US president, Joe Biden, declined Ukrainian requests for the lightweight fighter aircraft. Volodymyr Zelenskiy, Ukraine’s president, is understood to have subsequently given firm commitments that the planes would be used only to target Russian forces within Ukraine. It will still take months to train Ukraine’s pilots, who will have previously flown mainly in Soviet-standard aircraft. Ukraine does not yet even have runways suitable for F-16s should Nato allies agree to provide the hardware. Kyiv has in recent days warned that its long-anticipated counteroffensive is running into Russia’s air and artillery superiority. Despite those challenges, Brig Gen Oleksii Hromov said on Thursday that progress in its counteroffensive was being made, with Ukraine regaining control over 4o sq miles (103 sq km) since the weekend. Ukrainian forces were said to have pushed 2 to 4 miles into formerly occupied lands. Hromov told journalists that his troops had benefited from the provision of MiG-29 planes from Poland and Slovakia and claimed, without providing evidence, that Russian soldiers were deliberately inflicting wounds upon themselves to avoid battle. Hromov said: “The Ukrainian army has gained considerable experience and skill on the battlefield this year, and we are ready and will continue to fight until the complete liberation of our own territories, even with our bare hands. With heavy weapons from our allies, this will happen faster.” Ukraine’s army is said to have advanced by 2 miles near the village of Mala Tokmachka in the Zaporizhzhia region and by up to 4 miles near a village south of Velyka Novosilka in Donetsk. The Ukrainian deputy defence minister Hanna Maliar said: “There is a gradual but steady advance of the armed forces. At the same time, the enemy is putting up powerful resistance [on the southern front]. The enemy is pulling up additional reserves and is trying with all its might to prevent the advance of Ukrainian forces.” Stoltenberg said western hardware was making a difference in the fighting. Moscow has widely disseminated video footage of US Bradley armoured vehicles and German-made Leopard tanks being hit by Russian fire. The US defence secretary, Lloyd Austin, said Ukraine retained sufficient firepower for the battle to come. “I think the Russians have shown us that same five vehicles about 1,000 times from 10 different angles,” he said. “But quite frankly, the Ukrainians still have a lot of combat capability, combat power. “This will continue to be a tough fight, as we anticipate it, and I believe that the element that does the best in terms of sustainment will probably have the advantage at the end of the day.” The US chair of the joint chiefs of staff, Gen Mark Milley, said it was too early “to put any estimates” on how long the Ukrainian counteroffensive would last. He said: “This is a very difficult fight; it is a very violent fight and it will likely take a considerable amount of time and at high cost.” Russian military bloggers have claimed as many as 100 Russian troops gathered for a motivational speech near Ukraine’s eastern frontline may have been killed in a strike by a US-made Himars rocket system earlier this week, although Ukrainian officials declined to comment. Beyond the provision of fighter jet training, it has been announced in Brussels that the US, UK, the Netherlands and Denmark were sending “high priority” air defence equipment including hundreds of missiles to Ukraine and that the delivery would be completed within weeks. Last month, it emerged the UK had already delivered multiple Storm Shadow cruise missiles, giving Ukraine’s defence a new long-range strike capability. That move had prompted threats from the Kremlin and earlier this week Belarus’s leader, Alexander Lukashenko, engaged in further nuclear sabre-rattling as he announced that Russian tactical warheads would be relocated to Belarusian territory. Stoltenberg said the situation was being monitored but that it did not appear to be a significant move. “We are, of course, closely monitoring what Russia is doing,” he said. “So far, we haven’t seen any changes in the nuclear posture that requires any changes in our posture”. He added: “Russia’s nuclear rhetoric and messaging is reckless and dangerous. Russia must know that a nuclear war cannot be won and must never be fought.” Rafael Grossi, the head of the International Atomic Energy Agency, arrived in the south-eastern region of Zaporizhzhia to assess the risks to Europe’s biggest nuclear power plant following the destruction of the Kakhovka dam. The dam’s reservoir provides the cooling water for the plant. After his inspection, Grossi said measures had been taken to stabilise the situation but he added that inspectors would stay at the facility, in a sign of the concerns the visit had raised among officials at the UN’s atomic watchdog.