Russian President Vladimir Putin said on Friday that Ukrainian forces had certainly begun their expected counteroffensive in intense fighting in Ukraine, but that every attempted advance had failed, at a heavy cost in casualties.

His comments expanded a narrative that Russia has been developing since the start of the week, which Kyiv has declined to challenge, saying merely that the start of the counteroffensive will not be announced.

Russia's Defence Ministry earlier said it had repelled fierce attacks in the Zaporizhzhia and Donetsk regions, causing more than 1,000 Ukrainian casualties and destroying dozens of tanks and armoured vehicles, but it offered no evidence for its assertions.

It said Ukrainian forces had attacked Russian lines four times with two battalions supported by tanks just south of Velyka Novosilka in Donetsk, but were pushed back.

Russian forces also repelled two attacks just south of the city of Orikhiv in the Zaporizhzhia region, the ministry said.

Putin said Ukraine's casualties significantly exceeded the classical ratio of three attackers to one defender.

"All counteroffensive attempts made so far have failed. But the offensive potential of the troops of the Kyiv regime is still preserved," he added.

The ministry said Ukraine had lost around 1,200 men, some 40 tanks and several aircraft, including a MiG-29 and an Su-25, in the space of 24 hours, but did not detail Russia's losses.

Reuters cannot independently verify battlefield accounts or losses quoted by either side, both of which have in the past given high casualty figures for the other.

On Tuesday, Russia said 3,715 Ukrainians had been killed or wounded in three days, without explaining how it had arrived at such a precise tally.

Tens of thousands of Russian troops have for months been digging in along a front line stretching around 600 miles (1,000 km), bracing for an attack that is expected to try to cut Russia's so-called land bridge to the Crimean peninsula, which Moscow annexed in 2014.

The fate of the counteroffensive, supported by tens of billions of dollars in Western weapons, is likely to influence the shape of future Western diplomatic and military support for Ukraine.