In separate comments published on Sunday, Stoltenberg and Johnson also reiterated that Western governments must continue to support Ukraine to deter future aggression by Russian President Vladimir Putin.
Stoltenberg told German newspaper Bild am Sonntag that no one knew how long the conflict would last but “we have to be prepared for the fact that it could take years”.
“We must not stop supporting Ukraine. Even if the costs are high, not only for military support, but also because of rising energy and food prices.”
Johnson said taking over all of Ukraine’s Donbass, which covers much of eastern Ukraine, had been Putin’s goal for eight years “when he unleashed a separatist rebellion and launched his first invasion “.
While Russia was still falling short of this goal, “Putin may not realize it, but his grand imperial design for the total reconquest of Ukraine has been derailed. In his isolation, he may still think that ‘Total conquest is possible.’
Both men stressed the need to avoid future Russian aggression.
Stoltenberg said: “If Putin learns the lesson from this war that he can just continue as he did after the Georgia war in 2008 and the occupation of Crimea in 2014, then we will pay a much higher price. .”
Johnson asked what would happen if President Putin was free to keep all areas of Ukraine now controlled by Russian forces. “What if no one was willing to lift a finger as they annexed this conquered territory and its fearful people into a greater Russia? Would that bring peace?”
Johnson said that with strong, long-term support for Ukraine, “we and our allies will protect our own security as much as Ukraine’s and protect the world from Putin’s murderous dreams and those who may seek to destroy them.” to copy”.
Johnson wrote: “Time is the vital factor. Everything will depend on whether Ukraine can build up its ability to defend its soil faster than Russia can renew its ability to attack. Our task is to mobilize time from the Ukrainian side.”
On Sunday, Ukrainian officials said heavy fighting continued in the city of Severodonetsk – the epicenter of the bloody battle for Ukraine’s eastern Donbass region – and surrounding communities as Russian forces tried to break down resistance from Ukrainian defenders and capture parts of eastern Luhansk. region they do not already control.
Serhii Hayday, head of the regional military administration, said “the battles for Severodonetsk continue” and the sprawling Azot chemical plant, where some 500 civilians are sheltering, was again bombed.
Russian operations appear designed to break through Ukrainian defenses south of Lysychansk and Severodonetsk, cutting off Ukrainian units still defending the two strategically important cities.
To the west, in the Donetsk region, also in the Donbass, the Ukrainian army reported further shelling of Ukrainian positions near Sloviansk. There was also a missile strike in the area, according to an operational update from the Ukrainian General Staff. But there appears to have been little change in frontline positions.
Stoltenberg was cautiously optimistic that Ukraine could turn the tide of the war. “Although the battle in the Donbass is being waged more and more brutally by Russia, the Ukrainian soldiers are fighting valiantly. With more modern weapons, the likelihood increases that Ukraine can once again drive Putin’s troops out of the Donbass. ”
US officials insist Western weapons continue to flow to the front lines of the fight. But local reports of arms shortages – and frustrated pleas from frontline Ukrainian officials – have raised questions about the effectiveness of supply lines.
As Russia has made further gains in eastern Ukraine, Johnson pointed to the attrition of Russian forces in hard-fought battles, saying Russia would need “years, if not decades, to replace this material”. And hour by hour, the Russian forces are spending equipment and ammunition faster. that their factories cannot produce them.”
The British Prime Minister added: “The UK and our friends must respond by ensuring that Ukraine has the strategic stamina to survive and ultimately prevail.”
He outlined four essential steps to support Ukraine, including: preserving the Ukrainian state, which includes: ensuring the country receives “weapons, equipment, ammunition and training faster than the invader and strengthen its ability to use our aid; a “long-term effort to develop” alternative land routes to overcome “Russia’s stranglehold on Ukraine’s economy by blocking its main export routes through the Black Sea”.
This weekend, Zelensky traveled to the front lines of the coastal city of Odessa and the southern city of Mykolaiv, both of which are Russian targets in its bid to seize the sea coast Black.
Johnson added that Russia’s blockade of Black Sea ports meant that some “25 million tonnes of corn and wheat – the total annual consumption of all least developed countries – are piled up in silos across Ukraine”. .
On the upcoming NATO summit in Madrid, Stoltenberg said a new strategic concept will be adopted “will declare that Russia is no longer a partner, but a threat to our security, peace and stability.”
He said “Russia’s nuclear rattling is dangerous and irresponsible. Putin must know that a nuclear war cannot be won and must never be fought.”
CNN’s Tara John, Barbara Starr, Jeremy Herb and Oren Liebermann contributed to this article.