• It's been almost four months since Russia invaded Ukraine. 
• NATO's chief says the world should brace itself for a war that could last years. 
• He called for more support of Ukraine, adding that a win for Russia would be bad. 

NATO's Secretary-General Jens Stoltenberg said the war in Ukraine could go on for years. 

"We must prepare for the fact that it could take years. We must not let up in supporting Ukraine," Stoltenberg said during an interview with German newspaper Bild.

He added: "Even if the costs are high, not only for military support, but also because of rising energy and food prices. But: That is no comparison to the price that the Ukrainians have to pay every day with many lives."

Stoltenberg said while the cost of the war is great, letting Russian President Vladimir Putin win was even worse. 

He said continuing to support Ukraine militarily would help give them a better chance at winning the war. 

Russia invaded Ukraine almost four months ago, and with the help of western nations, Ukraine has been able to defend some of its land and hold Russian forces back. 

Stoltenberg's comments come as UK Prime Minister Boris Johnson also warned the war could continue for a while. 

In an op-ed in the Sunday Times, Johnson called on Western allies not to stop their support of Ukraine because a win from Russia would be "the greatest victory for aggression in Europe since the Second World War."

"We know Putin would not stop at dismembering Ukraine. Only last week, he compared himself to Peter the Great and arrogated to Russia an eternal right to "take back" any territory ever inhabited by "Slavs", a doctrine that would permit the conquest of vast expanses of Europe, including NATO allies," Johnson wrote.