Reports of a Ukrainian breakthrough on the front line of the war have been denied by Russia - even though the claim was made by pro-Russia sources.
Yevgeny Prigozhin, head of the Wagner mercenary group fighting on the Russian side, accused regular Russian troops of abandoning positions around Bakhmut.
And Russian military bloggers reported Ukrainian advances or troop movements in several areas on Thursday.
But the Kremlin denied Ukraine had made any significant advances.
In a statement, Russia's defence ministry said: "The individual declarations on Telegram about a 'breakthrough' on several points on the frontline do not correspond to reality."
"The general situation in the special military operation zone is under control," it added, using Russia's term for the invasion.
The Institute for the Study of War said Ukrainian forces had probably made gains in the eastern city of Bakhmut, forcing Russian troops back in some areas of the front by around two kilometres (1.2 miles).
And the US-based think tank said the Russian defence ministry acknowledging "the Ukrainian counterattacks uncharacteristically quickly" - by denying them - indicated "increased panic".
On Friday morning, the exiled mayor of Melitopol reported a large explosion in the centre of the city in south-eastern Ukraine, which has been occupied by Russia since start of the war.
It's not known what caused the blast. But the Ukrainian air force made 14 strikes on Russian forces and military equipment on Thursday, Ukraine's armed forces said.
Alongside the air strikes, Ukraine said it destroyed nine Russian drones and carried out successful attacks on dozens of military targets - including artillery units, an ammunition warehouse and air defence equipment.
Ukraine's president said on Thursday it was too early to start a counteroffensive.
"With [what we already have] we can go forward and, I think, be successful," President Volodymyr Zelensky said in an interview for public service broadcasters who are members of Eurovision News, like the BBC.
"But we'd lose a lot of people. I think that's unacceptable. So we need to wait. We still need a bit more time."
The Wagner boss Prigozhin - who has repeatedly clashed with Russian authorities - said the situation "on the flanks" at Bakhmut was "developing in line with the worst of the predicted scenarios".
"The territories that we have been taking for many months at the cost of the blood and lives of our brothers-in-arms, covering dozens or hundreds of metres a day, are now being abandoned, practically without a fight, by those who should be holding our flanks," he said.
Pro-Kremlin Russian war correspondent Sasha Kots claimed that Kyiv's much-anticipated counteroffensive had begun.
Ukrainian tanks were on the Kharkiv ring road heading towards the border with Russia, he said, quoting "trusted" sources. His claims could not be independently verified.
"There are low loaders in the columns carrying Western [tank] models among others," Kots added.
"In other words," he said, "Kiev [Kyiv] has decided to aggravate the situation along the northern front in parallel with the start of offensive actions on the flanks of Artyomovsk [Bakhmut]."
Another Russian war correspondent, Alexander Simonov, wrote on Telegram that Ukrainian forces had broken through near the village of Bohdanivka, close to Bakhmut, taking "several square kilometres" of ground.
Ukrainian military analyst Oleksandr Musivenko said Kyiv recognised that the anticipated counteroffensive might not necessarily defeat Russia "in all occupied areas".
He told Ukrainian NV radio there was every possibility the war could continue into next year.
"It all depends on how the battles develop. We can't guarantee how the counteroffensive will develop," he said.
An unnamed senior US military official told CNN that Ukrainian forces were preparing for a major counteroffensive by striking targets such as weapons depots, command centres and armour and artillery systems to prepare the field for advancing forces.
Ukraine's spring 2022 counteroffensive in the southern and north-eastern parts of the country was also preceded by air attacks to "shape" the battlefield.
But the shaping operations could take many days before the planned offensive gets underway, CNN quoted the senior US military official as saying.
Western officials estimate between 20,000 and 30,000 Russian troops have been killed or wounded at Bakhmut, while Ukraine's military has also paid a heavy price.
Russia's defence ministry also said it had stopped several Ukrainian attacks throughout Thursday and said an ongoing battle near Malynivka, in eastern Donetsk, involved both air and artillery forces.
Since Russia invaded Ukraine on 24 February of last year, thousands of civilians and combatants have been killed or injured, cities and towns have been destroyed in fighting, and nearly 8.2 million Ukrainians have been registered as refugees in Europe, with 2.8 million of them in Russia.