The war in Ukraine may be heading for its bloodiest phase yet, according to senior Ukrainian officials and western analysts, who say a long-threatened southern counteroffensive by Kyiv’s forces will result in a “huge battle” and will need to overcome increasingly well dug-in Russian troops.

People interviewed by the Guardian, including those who have recently escaped Kherson, describe Russian forces in the south building hardened tank shelters, digging trenches and bringing in prefabricated concrete pillboxes by lorry.An increase in strikes in recent days against key Russian weapons stores and logistics around Kherson – which fell to Russian forces in early March – has added to suggestions that some Ukrainian forces may be edging closer to the long-promised offensive, even as Russian officials say they plan to annex further territory. At his quarters in a training area in the south of the country hidden in an area of dense scrub, “Makita” – a Ukrainian battalion infantry instructor who only wished to give his nom de guerre – spelled out the challenges for a southern offensive. 

“We are holding our positions right now to the east of Mykolaiv. But the intention is to liberate the areas held by Russian forces. The biggest challenge is to do that with the minimum losses on the Ukrainian side.

“Russian forces are fortifying what are already good positions,” he added. “They had a month in which to improve the defences. They also appear to be bringing up reserves and more weapons including 2S7s [self-propelled heavy artillery].

“The first task will be to push back the Russians towards Kherson and over the Dnipro [Dnieper] River. We know it is going to be hard.”

With the far western end of the sprawling 120-mile southern frontline marked by the city of Mykolaiv, any Ukrainian offensive will be seen as a key test of whether Kyiv’s troops can push back in force, in contrast to the limited and local counteroffensives that have been witnessed so far.

The Russian foreign minister, Sergei Lavrov’s declaration this week that Russia was expanding its war aims from the Donbas, including to the Kherson and Zaporizhzhia regions in the south, has increased pressure on Kyiv to follow through on its promise to drive Russian forces from areas they have occupied.