Ukrainian forces made a major effort to punch through Russian defensive lines for a second day on Monday, as military analysts said the long-awaited counter-offensive had begun.
Ukrainian President Volodymyr Zelenskyy on Monday welcomed "the news we have been waiting for" from troops fighting in and around the shattered eastern city of Bakhmut, but gave no further details.
"I am grateful to each soldier, to all our defenders, men and women, who have given us today the news we have been waiting for. Fine job, soldiers in the Bakhmut sector," Mr Zelenskyy said in his nightly video address.
He said Russia was reacting "hysterically" to any action undertaken by Ukrainian forces and singled out two units who "skilfully, decisively and effectively defend our positions, destroy the occupiers and, most importantly, move forward".
Vladimir Rogov, an official in the Russia-backed administration of Ukraine's partly occupied Zaporizhzhia province, claimed fighting resumed there earlier in the day after Russian forces beat back a Ukrainian advance on Sunday.
“The enemy threw an even bigger force into the attack than yesterday,” and the new attempt to break through the front line was “more large-scale and organised", Mr Rogov said. “A battle is under way.”
Earlier, Russia's Defence Ministry said that its forces pushed back a “large scale” assault on Sunday at five points in southern Ukraine's Donetsk province.
Ukraine's Centre for Strategic Communications did not address the Russian statement directly but said Moscow would try to spread lies.
But Prof Michael Clarke, former director general of the Royal United Services Institute, said there was increased activity in eastern Ukraine, which 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 to push back Russian forces.
“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.”
Prof Clarke said several scenarios were possible, including a series of “diversionary activities” before the start of a more focused push.
“Another operation would be to have a series of pushes. It’s as if they would push on half a dozen doors, and whichever door seems to open the widest, most easily, then attracts the heavy metal to go towards it.
“A third possibility is a series of fingers of offensives, which sort of coalesce into something bigger. And we won’t know how that might develop until maybe next week or the week after.
“But clearly the reports that have been coming in overnight is that someone in Kyiv is now giving the orders for at least a series of local offensives or local pushes. Mainly, it seems at the moment, in the Donbas.”
On Monday, Russian mercenary leader Yevgeny Prigozhin said Ukrainian forces had retaken part of the settlement of Berkhivka, north of Bakhmut in eastern Ukraine, calling it a “disgrace”.
Mr Prigozhin's private Wagner army captured Bakhmut last month after the longest battle of the war and handed its positions there to regular Russian troops.
“Now part of the settlement of Berkhivka has already been lost, the troops are quietly running away. Disgrace!", Prigozhin said in an audio message published by his press service.
His comments came as an energy plant in Russia's Belgorod region was struck in the early hours of Monday after a drone attack, according to the region's governor.
“The preliminary cause of the fire was an explosive device dropped from a drone,” Vyacheslav Gladkov reported on the Telegram messaging app.
Mr Gladkov said the incident did not result in any casualties.
The interceptions are part of a series of near-daily attacks reported inside Russia or on Moscow-held territories.
Russian officials claimed that of the nine drones intercepted over the Crimean Peninsula, five were shot down and four were jammed, stopping them reaching their targets in Dzhankoi.
Another drone was later intercepted over Sudzha, as reported by the governor of the Kursk region on the Telegram messaging app.
Reuters was unable to independently verify these incidents.
The claimed drone interceptions follow a week of attacks on Russia's border region of Belgorod, the country's oil infrastructure, and a drone strike on affluent districts in Moscow.
Despite these incidents, Kyiv has denied attacking Moscow and rarely acknowledges responsibility for attacks elsewhere inside Russia or on Russian-controlled territories inside Ukraine.
Sunday's drone attacks resulted in property damage but no casualties, said Sergei Aksyonov, the Russian-backed head of Crimea's administration.
Several houses in Dzhankoi had broken windows, and a drone that failed to explode was discovered on the property of a residential house, leading to the temporary evacuation of about 50 people.
Ukrainian officials have previously said Moscow has turned Dzhankoi and its surrounding areas into the largest military base in Crimea.
Russia claimed it had thwarted a major Ukrainian offensive in the southern region of Donetsk on Sunday, resulting in “hundreds” of pro-Kyiv troop casualties.
The Russian defence ministry said early on Monday that Ukraine had launched the offensive on Sunday using six mechanised and two tank battalions.
“The enemy's goal was to break through our defences in the most vulnerable, in its opinion, sector of the front,” the Russian defence ministry said.
Russia claimed to have eliminated 250 Ukrainian troops and destroyed 16 tanks, infantry fighting vehicles, and 21 armoured combat vehicles.
Russia's Chief of General Staff, Valery Gerasimov, was reportedly in the area of the Ukrainian attack at the time.
Ukraine is aiming to regain territory seized by Russian forces since the February 2022 invasion.
Last week the former head of US forces in Afghanistan told The National Ukraine’s western allies risked a “suboptimal outcome” if they pressure Kyiv into launching the operation to push back Russian forces too soon.
“Leave them alone. Let them do this when they’re ready,” Gen John Allen, who also led the Nato international security assistance force, said on Tuesday on the sidelines of a defence forum in Slovakia’s capital Bratislava.
The military operation had been delayed by wet weather that has made the ground too soft for heavy fighting vehicles.
But recent drier conditions in parts of Ukraine means the conditions are now more favourable.
“We strongly believe that we will succeed,” President Zelenskyy told the Wall Street Journal.
“I don’t know how long it will take. To be honest, it can go a variety of ways, completely different. But we are going to do it, and we are ready.”
On Saturday Ukraine's Deputy Defence Minister Volodymyr Harylov said plans for the counter-offensive were on track, despite an “unprecedented” wave of missile and drone attacks across the country in recent weeks.
Speaking on the sidelines of the Shangri-La Dialogue in Singapore, Asia's top security conference, he said Ukraine had faced repeated volleys of ballistic missiles from Russia in May, especially in urban centres including Kyiv.