Volodymyr Zelenskyy says his top military commanders have pledged to continue defending the besieged eastern city of Bakhmut in the face of intense Russian pressure.
The Ukrainian president said the generals agreed at a meeting to strengthen the defences and the top brass would "find the appropriate forces to help our guys" there.
It comes as footage circulating on social media purports to show an unarmed Ukrainian prisoner of war being shot and executed by Russian troops.
The man in a Ukrainian uniform smokes a cigarette and says "Glory to Ukraine" before a hail of bullets are fired at him.
Ukraine's minister of foreign affairs, Dmytro Kuleba, described the video as "horrific" and said that it was more proof the war was "genocidal".
He called on the International Criminal Court to launch an immediate investigation into the alleged shooting, branding it a "war crime" and adding that its perpetrators "must face justice".
Mr Zelenskyy said his advisers had unanimously agreed to press on with the fight in Bakhmut and "not to retreat" despite Russia trying to capture it for six months and thousands of its soldiers losing their lives in the process.
Moscow has targeted the embattled Donbas city with heavy shelling in a three-sided assault, but its forces have been unable to deliver a knockout blow which would enable them to seize it and finish off the resistance.
A few days ago, one of Mr Zelenskyy's advisers admitted Ukrainian troops might have to retreat and fall back to nearby positions.
But on Monday, the president's top adviser, Mykhailo Podolyak, said the forces had been grinding down the invaders, reinforcing their positions and training tens of thousands of military personnel for a possible counteroffensive.
The city's importance is mostly symbolic as analysts say it is not strategically key in the war and Russia capturing it would unlikely mark a turning point. But it would mean President Vladimir Putin could deliver some good news to his people back home.
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However, the Bakhmut battle has exposed Russian military shortcomings and bitter divisions.
Yevgeny Prigozhin, the millionaire owner of the Wagner mercenary group which has spearheaded the Bakhmut offensive, has been at loggerheads for months with the Russian defence ministry, accusing it of deliberately depriving his men of ammunition - an allegation it rejects.
On Friday, Mr Prigozhin said his units had "practically surrounded Bakhmut".
But in a video released on Saturday, he warned: "If Wagner retreats from Bakhmut now, the whole front will collapse. The situation will not be sweet for all military formations protecting Russian interests."
And on Monday he admitted Ukraine had built up its own forces in surrounding towns and areas to try to push Wagner out of Bakhmut and that he needed help to take the city for Russia.
"I'm knocking on all doors and sounding the alarm about ammunition and reinforcements, as well as the need to cover our flanks," Mr Prigozhin said in a statement released by his press service.
"If everyone is coordinated, without ambition, screw-ups and tantrums, and carries out this work, then we will block the armed forces of Ukraine. If not, then everyone will be screwed."
Meanwhile, Mr Putin is delaying the announcement of a second mobilisation amid fears of any unrest in Russia, according to US think tank the Institute for the Study of War (ISW).
Last September, Mr Putin ordered a partial mobilisation in Russia in what appeared to be an admission Moscow's war in Ukraine was not going to plan.
Defence minister Sergei Shoigu said at the time Russia would draft 300,000 reservists.
However, the ISW said Mr Putin has been "delaying announcing the second mobilisation wave since January and is reportedly doubling down on 'quiet mobilisation' to avoid generating possible unrest in Russia".