Nato-trained units and western tanks are spearheading Ukraine’s long-awaited ground offensive, according to images emerging from the front line.
Kyiv has maintained a strict silence about its long-anticipated counter-attack, but three senior US officials were quoted in reports on Thursday confirming the operation was under way.
With virtually no independent reporting from the battlefield, it is difficult to assess whether Ukraine's operation is in motion and if it is penetrating Russian defences to drive out occupying forces.
It came as Russia confirmed it will start deploying tactical nuclear weapons in Belarus after the facilities are ready on July 7-8, President Vladimir Putin told his Belarusian counterpart Alexander Lukashenko on Friday.
“So everything is according to plan, everything is stable,” Mr Putin said in a meeting in Sochi, according to a readout from the Kremlin.
The two men had previously agreed the plan to deploy Russian land-based short-range nuclear missiles on the territory of Moscow's close ally, where they will remain under Russian command.
However, images posted by Russian military bloggers, which appear genuine, show Nato-standard battle tanks and fighting vehicles pushing toward Tokmak in Ukraine’s occupied south.
Attacks were described as coming in waves, with heavy fighting reported in the Zaporizhzhia region near the city of Orikhiv.
In his nightly address, Ukraine President Volodymyr Zelenskyy said: “I am in constant communication with our military. The commanders of Khortytsia, Tayria, all those involved in the hottest areas."
He said there were "very tough" battles in the Donetsk region.
“But there are results and I am grateful to those who achieved these results. Well done in Bakhmut. Step by step," he said.
The country'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 says its main effort is yet to begin.
Pro-war Russian bloggers reported intense battles on Friday around the mid-point of the "land bridge" linking Russia to the Crimea peninsula, thought to be one of Ukraine's main potential targets.
Ukraine generally forbids independent accredited journalists from reporting on its side of the front lines during offensive operations.
The initial days of the counter-offensive have been overshadowed by a humanitarian disaster after the destruction of the Kakhovka dam holding back the waters of the Dnipro River that bisects Ukraine.
Thousands have been forced to evacuate flooded homes, while vast nature preserves have been wiped out and the destruction to irrigation systems is likely to cripple agriculture across much of southern Ukraine.
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.
Norsar, the Norwegian Seismic Array, said it had detected "clear signals" to indicate that "an explosion" occured at the time and location of reports in the media about the collapse of the Kakhovka dam, which was under Russian control.
In its latest report from the battlefield, the Russian army claimed to have destroyed more than 21 armoured vehicles in the past 24 hours, which could not be verified.
In its few comments, Ukraine has reported gains of territory in the east around the city of Bakhmut, which Russian forces captured last month.
But Kyiv has said little 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, Mr Zelenskyy said: "We see every detail. But it's not time to talk about it today."
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 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 by 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 region of Cherkasy on Thursday evening. Regional governor Ihor Taburets said at least eight were wounded.
Ukraine and Russia conflict latest - in pictures
Earlier this week Prof Michael Clarke, former director general of the Royal United Services Institute, said the increased activity in eastern Ukraine marked the start of ground operations in the long-awaited counter-offensive.
The commander of Ukraine's ground forces, Oleksandr Syrskyi, said on Monday that they continued “moving forward” near the long-contested city of Bakhmut in northern Donetsk.
Prof Clarke said the increased activity could be considered the beginning of the ground operation.
“Something has begun and I am not at all surprised," he told BBC Radio 4 on Monday.
“We thought it had to start soon and we will see how it develops.
"There are two or three ways in which it might develop but I think June 4 will go down as the day when the Ukrainians began their ground operation.”
