Ukraine's Foreign Minister Dmytro Kuleba says Russia's assault on Ukraine's eastern front is the "largest" seen in Europe since World War Two.
He described Russian attacks in the Donbas as "ruthless", and called on western allies on Tuesday to speed up vital deliveries of weapons.
Moscow is attempting to seize the region's two eastern provinces, Donetsk and Luhansk, and trap Ukrainian forces in a pocket on the main eastern front. The UK's defence ministry has said Moscow intends to occupy all of the Luhansk region.
The battles currently being fought in eastern Ukraine could determine the whole country's fate, Ukrainian Defence Ministry spokesman Oleksandr Motuzyanyk said on Tuesday.
The ministry's comments came as Russian forces conducted an all-out assault, in the easternmost part of the Ukrainian-held Donbas.
They are trying to encircle Ukrainian troops in twin cities straddling the Siverskiy Donets river, Sievierodonetsk on the east bank and Lysychansk on the west bank. Motuzyanyk said Russian forces had not given up attempts to cross it.
"Now we are observing the most active phase of the full-scale aggression which Russia unfolded against our country," he told a televised briefing.
"The situation on the (eastern) front is extremely difficult, because the fate of this country is perhaps being decided (there) right now."
"The enemy has focused its efforts on carrying out an offensive in order to encircle Lysychansk and Sievierodonetsk," said Serhiy Gaidai, governor of Luhansk province, where the two cities are among the last territory still held by Ukraine.
"The intensity of fire on Sievierodonetsk has increased by multiple times, they are simply destroying the city," he said on TV, adding there were about 15,000 people living there.
Euronews International Correspondent Anelise Borges, reporting from Dnipro, said fighting in the east had intensified since Russian forces took control of the port of Mariupol.
"Since that happened, there seems to be a renewed effort to break through Ukrainian lines of defence all across the Donbas, especially in cities like Sievierodonetsk where Ukrainian troops are said to still be holding ground," she said, adding that the Russians had nonetheless taken control of several towns and villages.
Comments by senior Russian officials on Tuesday suggested plans for a drawn-out conflict ahead. Defence Minister Sergei Shoigu said Russia was deliberately advancing slowly to avoid civilian casualties.
Nikolai Patrushev, head of Putin's security council, said Moscow was not "chasing deadlines" and would fight as long as necessary to eradicate "Nazism" in Ukraine, a baseless claim that has echoes of Nazi Germany's own claims to be acting in self-defence when it sparked World War II by attacking its neighbours.
Three months into its war on Ukraine, Vladimir Putin still has only limited gains to show for his country's worst military losses in decades.
Meanwhile, much of Ukraine has suffered devastation. Around 6.5 million people have fled abroad, uncounted thousands have been killed and cities have been reduced to rubble.
In his video address on Monday night, Ukrainian President Volodymyr Zelenskyy said Russia was waging “total war” on his country, and that included inflicting as many casualties and as much infrastructure destruction as possible.
Watch Anelise Borges reporting from Ukraine in the video player above.