In separate comments published 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 the German newspaper Bild am Sonntag that nobody knew how long the conflict would last but “we need to prepare for the fact that it could take years.”
“We must not cease to support Ukraine. Even if the costs are high, not only for military support, but also because of rising energy and food prices.”
Johnson said that seizing all of Ukraine’s Donbas, which covers much of eastern Ukraine, had been Putin’s objective for the last eight years “when he ignited a separatist rebellion and launched his first invasion.”
While Russia was still short of this goal, “Putin may not realise it but his grand imperial design for the total reconquest of Ukraine has been derailed. In his isolation, he may still think total conquest is possible.”
Both men stressed the need to avert future Russian aggression.
Stoltenberg said: “If Putin learns the lesson from this war that he can just carry on 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 the areas of Ukraine now controlled by Russian forces. “What if no one was willing to lift a finger as he annexed this conquered territory and its fearful people into a greater Russia? Would this bring peace?”
Johnson said that through firm long-term support for Ukraine, “we and our allies will be protecting our own security as much as Ukraine’s and safeguarding the world from the lethal dreams of Putin and those who might seek to copy them.”
Johnson wrote: “Time is the vital factor. Everything will depend on whether Ukraine can strengthen its ability to defend its soil faster than Russia can renew its capacity to attack. Our task is to enlist time on Ukraine’s side.”
On Sunday, Ukrainian officials said heavy fighting continues in the city of Severodonetsk — the epicenter of the bloody battle for Ukraine’s eastern Donbas region — and surrounding communities as Russian forces try to break the resistance of Ukrainian defenders and capture parts of the eastern Luhansk region they don’t already control.
Serhii Hayday, head of the regional military administration, said the “battles for Severodonetsk continue,” and that the sprawling Azot chemical plant, where some 500 civilians are sheltering, had been shelled again.
Russian operations appear designed to break Ukrainian defenses to the 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 Donbas, the Ukrainian military reported further shelling of Ukrainian positions near Sloviansk. There was also a missile strike in the area, according to an operational update by 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 Donbass is being waged more and more brutally by Russia, Ukrainian soldiers are fighting valiantly. With more modern weapons, the probability increases that Ukraine will be able to drive Putin’s troops out of Donbas again.”
US officials insist that Western arms are still flowing to the front lines of the fight. But local reports of weapons shortages — and frustrated pleas from Ukrainian officials on the front lines — have raised questions about how effectively supply lines are running.
While Russia has been making incremental gains in eastern Ukraine, Johnson stressed the attrition of Russian forces in the grinding battles, saying Russia would need “years, perhaps decades, to replace this hardware. And hour by hour Russian forces are expending equipment and ammunition faster than their factories can produce them.”
The British Prime Minister added: “The UK and our friends must respond by ensuring that Ukraine has the strategic endurance to survive and eventually prevail.”
He laid out four essential steps to support Ukraine, which included: preserving the Ukrainian state which includes: ensuring the country receives “weapons, equipment, ammunition and training more rapidly than the invader and build up its capacity to use our help;” a “long-term effort to develop” alternative overland routes to overcome Russia’s “stranglehold on Ukraine’s economy by blockading its principal export routes across the Black Sea.”
This weekend, Zelensky visited the frontlines in the coastal city of Odesa and southern city of Mykolaiv, which are both Russian targets in its attempt to seize the Black Sea coast.
Johnson added that Russian blockade of Black Sea ports meant that some “25 million tonnes of corn and wheat — the entire annual consumption of all the least developed countries — is piled up in silos across Ukraine.”
On the forthcoming NATO summit in Madrid, Stoltenberg said that a new strategy concept will be adopted “will declare that Russia is no longer a partner, but a threat to our security, peace and stability.”
He said that “Russia’s nuclear saber rattling is dangerous and irresponsible. Putin must know that a nuclear war cannot be won and must never be waged.”
CNN’s Tara John, Barbara Starr, Jeremy Herb and Oren Liebermann contributed to this piece.