Pro-Moscow separatists say they have taken control of Lysychansk, Ukraine's last bastion in the eastern province of Luhansk.
Rodion Miroshnik, ambassador to Russia of the pro-Moscow Luhansk People's Republic, told Russian television: "Lysychansk has been brought under control. Unfortunately, it is not yet liberated."
Russian media showed footage of Luhansk militia fighters marching in the streets, waving flags and cheering.
But Ukraine National Guard spokesman Ruslan Muzychuk told Ukrainian television the city is still under Ukrainian control.
"Now there are fierce battles near Lysychansk, however, fortunately, the city is not surrounded and is under the control of the Ukrainian army," he said.
It comes just days after Russian troops took Lysychansk's sister city Severodonetsk, on the opposite side of the Siverskiy Donets river.
Oleksiy Arestovych, an adviser to Ukrainian President Volodymyr Zelenskyy, said that Russian soldiers had crossed the river and were coming towards Lysychansk from the north.
He said: "This is indeed a threat. We shall see.
"I do not rule out any one of a number of outcomes here.
"Things will become much more clear within a day or two."
But Mr Arestovych also said that if Russia takes Lysychansk, it will find its forces spread very thinly, as it tries to keep control of the six major cities it has taken in the Donbas, a region made up of Luhansk and neighbouring Donetsk.
He added: "The more Western weapons come to the front, the more the picture changes in favour of Ukraine."
• In his nightly address, Mr Zelenskyy praised those defending Ukraine and called for the truth about the war to be spread
• Eight Ukrainian city and village leaders are being held captive in Russia, according to The Association of Cities of Ukraine
• Ukrainian forces have repelled Russian advances in Novomykhailivka, Bohorodychne, and Ivanivka
• A Russian mine detonated, killing one civilian and injuring another in the Odesa region
• Belarusian President Alexander Lukashenko claimed Ukraine tried to strike military facilities on Belarusian territory three days ago, although he provided no evidence
Meanwhile, Russia said it had hit army command posts in Mykolaiv, near the Black sea port of Odesa, where powerful explosions were also reported.
At least three people were killed in Belgorod, a Russian city near the Ukrainian border, in the early hours of Sunday.
Vyacheslav Gladkov, the governor of the Belgorod region, said that at least 11 apartment buildings and 39 houses were damaged.
Ukraine has not commented on the reports.
It comes after a Russian attack on an apartment block near Odesa on Friday, which killed at least 21 people, and on a shopping centre in the central city of Kremenchuk on Monday, which killed at least 19.