Ukraine said on Friday it had retaken swathes of ground near the frontline city of Bakhmut, as Russia insisted it had repelled an attack along a broad stretch of the front line.
Yevgeny Prigozhin, the head of the Wagner mercenary group, who is spearheading Moscow's attack on Bakhmut, accused the Russian army of "fleeing" from around the eastern Ukraine city.
The conflicting reports from the battlefront indicate an increase in fighting after months of relative stability, as expectations grow over Kyiv's spring offensive.
Moscow also denounced as "extremely hostile" a British decision to send long-range missiles to Ukraine.
The question of when and where Ukraine might launch its high stakes fightback has been the subject of steady speculation, even as President Volodymr Zelensky insisted earlier this week that his army needed more time to prepare.
The fresh fighting came as China said it would send a special envoy to Europe next week, as Beijing pursues efforts to cast itself as peacemaker.
China plans to send high-ranking diplomat Li Hui to Ukraine, Poland, France, Germany and Russia to "communicate with all parties on the political settlement of the Ukraine crisis".
From Ukraine to the Middle East, Beijing in recent months has sought to position itself as a mediator with a leading role in solving the world's crises.
But while China presents itself as a neutral party on the Ukraine war, it has been criticised in the West for refusing to condemn Moscow for the invasion.
Zelensky is expected in Rome on Saturday for talks with political leaders and possibly Pope Francis, his first visit to Italy since Russia's invasion.
'Great losses' in Bakhmut fight
On the battlefield, Ukraine said its forces had advanced 2km near Bakhmut – the scene of the longest and bloodiest battle since Moscow's more than year-long invasion.
Bakhmut, which once had a population of around 70,000 people, has been destroyed as Russian forces posted incremental gains in recent months, amounting to some 80% of the city.
Moscow has denied Ukraine had made any breakthroughs there, adding on Friday that it had repelled Kyiv's forces along more than 95km of front near the eastern town of Soledar.
Russia's defence ministry said Ukraine had deployed more than 1,000 military personnel and up to 40 tanks.
Wagner chief Prigozhin however said Moscow's conventional army "simply went fleeing from the flanks" near Bakhmut.
Prigozhin insisted that "the flanks are failing, the front is collapsing" in that area and said Russia's leadership was downplaying the gravity of the situation.
"For this reason, we must stop lying immediately," Prigozhin said in a video statement released on social media.
The social media accounts of several Russian war correspondents also expressed alarm late on Thursday, with some saying Kyiv's long-anticipated counteroffensive had begun.
Zelensky, however, said in an interview published on Thursday that Kyiv needed more equipment before going on the offensive.
But in his evening address on Friday, he said of the Russian forces: "The occupiers are already internally ready for defeat.
"They have already lost this war in their minds."
Prigozhin, whose long-running feud with Russia's conventional army has flared in recent days, acknowledged recent Ukrainian successes.
He even challenged his rival, Defence Minister Sergei Shoigu, to visit Bakhmut.
"The enemy has launched a number of successful counterattacks," Prigozhin wrote to Shoigu on social media, urging him "to independently assess the current situation".
Moscow denounces UK over missiles
Moscow on Friday denounced the UK's decision to deliver long-range Storm Shadow missiles to Ukraine, announced a day earlier.
"We see this decision as an extremely hostile step from London, aimed at further pumping weapons into Ukraine and leading to a serious escalation of the situation," said a foreign ministry statement.
Russia said it would "take all necessary measures to neutralise the threats" posed by the new missiles.
The Kremlin announced on Friday that Russian President Vladimir Putin had agreed to deepen ties with South African leader Cyril Ramaphosa – just a day after the US accused Pretoria of secretly shipping arms to Russia.
Ramaphosa has already reacted angrily to Thursday's accusation from US ambassador Reuben Brigety, while announcing an investigation into the affair to be led by a retired judge.
Turkey said on Friday that talks to extend a deal allowing grain exports from Ukraine via the Black Sea following the Russian invasion were nearing an agreement.
"We are heading toward an agreement on the extension of the grain deal," Turkish Defence Minister Hulusi Akar said in a statement.
The Black Sea Grain Initiative, in place since July after diplomacy by the United Nations and Turkey, allows Ukraine grain exports via port, helping ease shortages and resulting price spikes triggered by Russia's invasion of the breadbasket nation.