Jens Stoltenberg, NATO’s secretary-general, told German newspaper Bild in an interview published Sunday that the world must be prepared for the war in Ukraine to last for years.

“We must not let up in supporting Ukraine, even if the costs are high—not only for military support, but also because of rising energy and food prices,” the Nato chief said.

“That is no comparison to the price that the Ukrainians have to pay every day with many lives. And if Putin learns from this war that he can simply carry on as he did after the 2008 war in Georgia and the occupation of Crimea in 2014, then we will pay a much higher price.”

Stoltenberg’s comments came as the new head of the British army said the U.K. and its allies needed to be “capable of … defeating Russia” in battle.

In an internal message made public yesterday by the BBC over the weekend, General Sir Patrick Sanders said: “Russia’s invasion of Ukraine underlines our core purpose—to protect the U.K. and to be ready to fight and win wars on land—and reinforces the requirement to deter Russian aggression with the threat of force.

“The world has changed since [Russia invaded Ukraine on] 24th February and there is now a burning imperative to forge an army capable of fighting alongside our allies and defeating Russia in battle,” he added.

Meanwhile, in an article for Britain’s Sunday Times newspaper, U.K. Prime Minister Boris Johnson called on world leaders to brace themselves to stand by Ukraine in the long-term.

“Time is now the vital factor,” he said. “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.”

Ukraine conceding territory to Russia “would be the greatest victory for aggression in Europe since the Second World War,” Johnson added.