KYIV: The Ukrainian military was preparing a counteroffensive on Sunday, with a top commander saying his forces' ongoing defense of Bakhmut in the face of fierce and sustained Russian attacks was necessary to 'buy time' for that push.

The remarks came as United Kingdom intelligence said the frontline had shifted in the fight for Bakhmut — the longest and bloodiest battle in Moscow's yearlong invasion — but that any further Russian advance in the devastated eastern city would be 'highly challenging.'

Some military experts have questioned the sense of continuing to hold the city, but Oleksandr Syrskyi, the commander of Ukraine's ground forces, said it helped win time in readying the counteroffensive.

'The real heroes now are the defenders who are holding the eastern front on their shoulders, and inflicting the heaviest possible losses, sparing neither themselves nor the enemy,' Syrskyi was quoted as saying in a statement on Saturday. 'It is necessary to buy time to build reserves and launch a counteroffensive, which is not far off.'

In a video on Saturday, Yevgeny Prigozhin, chief of the Russian mercenary group Wagner, said his forces were close to the city's administrative center.

Standing on the rooftop of a high-rise building in what is said to be Bakhmut, Prigozhin is seen pointing toward a building in the distance.

'This is the building of the town administration, this is the center of the town,' he said, clad in full military gear. 'It is 1 kilometer and 200 meters away.'

Speaking as artillery boomed in the background, Prigozhin said the most important thing now was to receive more ammunition from the army and 'move forward.'

Wagner has been spearheading offensives against cities in eastern Ukraine, including Bakhmut. Both sides have suffered heavy losses.

The Ukrainian Defense Ministry on Saturday reported that its forces had repelled 'more than 100 enemy attacks' over the last day along the eastern front.

'A killing zone'