KYIV, June 8 - A substantial Ukrainian force was pushing an assault against Russian positions in the south on Thursday, in an intensification of fighting that some Ukrainian officials and western analysts said marked the start in earnest of Kyiv’s much-vaunted counteroffensive.

The combat against Russian positions south of Zaporizhzhia included western-supplied tanks and armored vehicles and infantry backed by artillery, The Guardian reported. There were reports of intense fighting outside the town of Tokmak, a key Russian logistical hub.

Asked about US media reports that the counteroffensive had begun, a spokesperson for Ukraine’s general staff said: “We have no such information. And we do not comment on anonymous sources.”

Ukrainian officials, however, have insisted that they would not make an official announcement when the counteroffensive begins and have cautioned against focusing on a single area, suggesting it could involve a number of components.

The Institute for the Study of War, a US-based thinktank that has been producing daily updates on the war, said it had concluded on Thursday that the Ukrainian counteroffensive had begun.

“Activity throughout Ukraine is consistent with a variety of indicators that Ukrainian counteroffensive operations are under way across the theater,” the group tweeted.

On Thursday evening president Volodymyr Zelenskiy hailed what he described as “results” in heavy fighting in the Donetsk region.

“There is very heavy fighting in Donetsk region,” Zelenskiy said. “But there are results and I am grateful to those who achieved these results. Well done in Bakhmut. Step by step.”

Zelenskiy referred to other areas where fighting is going on, but said he would provide no details.

Following days of heavy shelling of Russian defenses, Ukrainian units were reported to have stepped up efforts to break thorough in the Orihiv-Tokmak sector, which blocks the route to the Russian-occupied city of Melitopol – a key objective for Kyiv.

Unconfirmed reports suggested that German-made Leopard tanks and US Bradleys had been thrown into the battle along with scores of other armored vehicles.

The Russian defense minister, Sergei Shoigu, said Ukrainian forces from Ukraine’s 47th mechanized brigade, with up to 1,500 troops and 150 armored vehicles, made “an attempt to break through” but Russian forces dealt the attack “a preventive blow”, although Russian military bloggers painted a far more nuanced picture.

Images posted by the Russian military blogger Rybar purported to show Ukrainian armor immobilized during the assault although neither those claims nor those of Shoigu could be verified.

Vladimir Rogov, a Russian occupation official in the area, also told the RIA Novosti news agency that Ukraine’s forces were exerting “maximum force” against Russian positions in the Zaporizhzhia region.

Russia has long anticipated a Ukrainian offensive in this direction, building multi-tiered defenses around this area of the frontline including zig-zag trenches, dragon’s teeth and other obstacles.

Claims that Kyiv is escalating efforts in the Zaporizhzhia area followed days of heavy shelling, with fires visible on Nasa’s Firms satellite monitoring system.

Appearing to confirm that a major assault was under way, the Ukrainian deputy defense minister, Hanna Maliar, said Russia was “actively on the defensive” in the Orihiv area of Zaporizhzhia region.

The fighting near Zaporizhzhia will be intensely scrutinized as this area of the front has long been seen as a potential axis for a Ukrainian assault towards Melitopol, not least because Tokmak itself is a major railway junction and Russian logistical hub that has key lines of communication with Mauripol, Berdyansk and Crimea.

Another objective of a Ukrainian assault south and east from this direction has long been supposed to sever the Russian “land bridge” linking areas of the occupied south and east with Crimea.

On 3 June, Zelensiky announced that the armed forces of Ukraine were ready for the long-awaited counteroffensive.

“I don’t know how long it will take,” he told the Wall Street Journal. “To be honest, it can go a variety of ways, completely different. But we are going to do it, and we are ready.”

Commenting on whether the destruction of the Kakhovka dam earlier this week might hinder Kyiv’s efforts, US analyst Michael Kofman said: “If the Ukrainian plan is to break through [Russian] lines in Zaporizhzhia and advance to the ground lines of communication from Crimea, or sever the ‘land bridge’ the resultant flooding is unlikely to impede such an operation.” (tg/ez)