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He said Russian forces are hitting cities in the east of the country with air and missile strikes.
Two cities, Lyman and Severodonetsk, are significant targets in Russia’s push to seize the Donbas region.
“We are protecting our land in a way that our current defence resources allow us. We are doing everything to reinforce it. And we will reinforce it,” he said.
“If the occupiers think that Lyman and Severodonetsk will be theirs, they are wrong. The Donbas will be Ukrainian because it’s us, it’s our essence.
“Even if Russia brings destruction and suffering there, we will rebuild every city and every community. There is no, and there won’t be any, real alternative to Ukrainian flags.”
He added: “As expected, the situation [in the Donbas] is extremely difficult.
“The occupiers are trying to achieve the goals they hoped to achieve in the first days of the war after February 24, at least before the 100th day of the war. That’s why they have concentrated maximum artillery and reserves [there].”
Ukrainian forces are doing everything possible to protect the territory, he said.
Severodonetsk and Lyman are the last areas under Ukrainian control in Luhansk, one of two provinces that make up the Donbas. Russia-backed separatists have controlled parts of the territory for the past eight years.
“Massive artillery shelling does not stop, day and night,” Severodonetsk Mayor Oleksandr Striuk said.
“The city is being systematically destroyed — 90 per cent of the buildings in the city are damaged.”
Mr Striuk described conditions as reminiscent of the battle for Mariupol, a port city located in the Donetsk region, which is now in ruins.
Russian forces are “chewing through ground” in Ukraine and closing in on capturing Severodonetsk, western officials said.
UK Prime Minister Boris Johnson said Ukraine's successful defence of Kyiv in the early weeks of the war should not lull western countries into thinking Russia was defeated.
“I’m afraid that [Russian President Vladimir] Putin, at great cost to himself and Russian military, is continuing to chew through ground in Donbas, he’s continuing to make gradual, slow, but I’m afraid palpable progress,” he told Bloomberg.
A US defence official said in a briefing that Russian troops had “essentially encircled” Severodonetsk and seized the city’s north-east, but that fighting continued despite efforts to stop Ukraine resupplying its forces.