(Reuters) - Russia reported heavy fighting along the front in southern and eastern Ukraine on Friday, while Kyiv maintained a strict silence about its long-anticipated counterattack.

With virtually no independent reporting from the front lines, it was impossible to assess the degree to which Ukraine's operation was under way or whether it was having success in penetrating Russian defences to drive out occupying forces.

Ukraine's counter-offensive is ultimately expected to involve thousands of Ukrainian troops trained and equipped by the West. Russia, which has had months to prepare its defensive lines, says it has withstood attacks since the start of the week. Kyiv has so far said its main effort has yet to begin.

Pro-war Russian bloggers reported intense battles on Friday on the Zaporizhzhia front near the city of Orikhiv, around the mid-point of the "land bridge" linking Russia to the Crimea peninsula, seen as one of Ukraine's main potential targets.

Ukraine generally forbids independent accredited journalists from reporting on its side of front lines during offensive operations.

The initial days of the counter-offensive have been overshadowed this week by a humanitarian disaster after the destruction of the Kakhovka dam holding back the waters of the Dnipro River that bisects Ukraine.

Thousands of people have been forced to evacuate homes flooded in the war zone, vast nature preserves have been wiped out and the destruction to irrigation systems is likely to cripple agriculture across much of southern Ukraine for decades.

Kyiv said on Friday it had intercepted a phone call proving that Russian forces in control of the dam had blown it up. Moscow says Ukraine sabotaged it. Western countries say they are still gathering evidence but that Ukraine would have no reason to inflict such a devastating disaster on itself, especially right as its forces were shifting onto the attack.

In its latest report from the battlefield, the Russian army claimed to have destroyed more than 21 armoured vehicles in the past 24 hours. Such claims are unverifiable.

In its few comments, Ukraine has reported gains of territory in the east around the city of Bakhmut, which Russian forces captured last month after nearly a year of the deadliest ground combat in Europe since World War Two.

But Kyiv has said virtually nothing about the southern front, widely assumed to be the focus of its main assault as it tries to push towards the coast and cut Russia's access to Crimea.

In his nightly video address, delivered on a train after a visit to the flood zone in the south, President Volodymyr Zelenskiy thanked Ukrainian troops and repeated earlier claims of success in Bakhmut, but gave no further account.

"We see every detail. But it's not time to talk about it today," he said.

Ukrainian Deputy Defence Minister Hanna Maliar described heavy fighting in the east, where she said Ukrainian troops had mainly held off Russian attacks.

On the southern front she said only that battles were continuing for the settlement of Velyka Novosilka and that Russian troops were mounting "active defence" at Orikhiv.

Ukraine has been attacking targets deep in Russian-held territory for weeks in preparation for its assault. Moscow has been striking Ukrainian cities with cruise missiles and drones.

In the latest air strikes, Ukraine said it had shot down four of six missiles overnight.

The interior ministry said one person had been killed, three were wounded, and four buildings were destroyed from falling debris. It posted images on Telegram of firefighters attending to the smouldering wreckage of what appeared to be residential homes.

The air force also said two cruise missiles had struck a civilian object in the central Ukrainian region of Cherkasy earlier on Thursday evening. Regional governor Ihor Taburets said at least eight people were wounded.