KYIV, Ukraine — Four months of brutal fighting in Ukraine appear to be straining the morale of troops on both sides, prompting desertions and rebellion against officers’ orders, British defense officials said Sunday. NATO’s chief warned the war could drag on for “years.”

“Combat units from both sides are committed to intense combat in the Donbas and are likely experiencing variable morale,” Britain’s defense ministry said in its daily assessment of the war, which has been raging for almost four months.

“Ukrainian forces have likely suffered desertions in recent weeks,” the assessment said, but added that “Russian morale highly likely remains especially troubled.”

It said “cases of whole Russian units refusing orders and armed stand-offs between officers and their troops continue to occur.”

The British note said that many Russian soldiers of all ranks “likely remain confused about the war’s objectives.”

In an interview published on Sunday in the German weekly Bild am Sonntag, NATO Secretary General Jens Stoltenberg said that “nobody knows” how long the war could last. “We need to be prepared for it to last for years,” he said.

He also urged allies “not to weaken support for Ukraine, even if the costs are high, not only in terms of military aid, but also because of the increase in energy and food goods prices.”

In recent days, Gazprom, the Russian gas company, has reduced supplies to two major European clients — Germany and Italy. In Italy’s case, energy officials are expected to huddle this week about the situation. The head of Italian energy giant ENI said on Saturday that with additional gas purchased from other sources, Italy should make it through the coming winter, but he warned Italians that “restrictions” affecting gas use might be necessary.

Germany will limit the use of gas for electricity production amid concerns about possible shortages caused by a reduction in supplies from Russia, the country’s economy minister said on Sunday. Germany has been trying to fill its gas storage facilities to capacity ahead of the cold winter months.

Economy Minister Robert Habeck said that Germany will try to compensate for the move by increasing the burning of coal, a more polluting fossil fuel. “That’s bitter, but it’s simply necessary in this situation to lower gas usage,” he said.

Stoltenberg stressed, though, that “the costs of food and fuel are nothing compared with those paid daily by the Ukrainians on the front line.”

Stoltenberg added: What’s more, if Russian President Vladimir Putin should reach his objectives in Ukraine, like when he annexed Crimea in 2014, “we would have to pay an even greater price.”

Britain’s defense ministry said that both Russia and Ukraine have continued to conduct heavy artillery bombardments on axes to the north, east and south of the Sieverodonetsk pocket, but with little change in the front line.

Luhansk governor Serhiy Haidai said via Telegram on Sunday: “It is a very difficult situation in Sievierodonetsk, where the enemy in the middle of the city is conducting round-the-clock aerial reconnaissance with drones, adjusting fire, quickly adjusting to our changes.”

On Saturday, Ukrainian President Volodymyr Zelenskyy made a trip south from Kyiv to visit troops and hospital workers in the Mykolaiv and Odesa regions along the Black Sea. He handed out awards to dozens of people at every stop, shaking their hands and thanking them again and again for their service.

Some time after Zelenskyy left Mykolaiv, “the enemy carried out fire damage against units of the Defense Forces with cannon and rocket artillery in the areas of the settlements of Pravdyne, Posad-Pokrovskoe and Blahodatne,” according to the Ukrainian army briefing on Sunday.

Stoltenberg’s comments were published a day after British Prime Minister Boris Johnson expressed concerns “that a bit of Ukraine fatigue is starting to set in around the world” and said Ukraine must be supported in trying to roll back the Russian invasion.

“It would be a catastrophe if Putin won. He’d love nothing more than to say, ‘Let’s freeze this conflict, let’s have a cease-fire,'” Johnson said.

Johnson spoke on his return from a surprise trip Friday to Kyiv where he met with Zelenskyy to offer continued aid and military training.

Western-supplied heavy weapons are reaching front lines. But Ukraine’s leaders have insisted for weeks that they need more arms and they need them sooner.