The head of NATO expressed worry that the fighting in Ukraine could spin out of control and become a war between Russia and NATO, according to an interview released Friday.

“If things go wrong, they can go horribly wrong,” NATO Secretary-General Jens Stoltenberg said in remarks to Norwegian broadcaster NRK.

“It is a terrible war in Ukraine. It is also a war that can become a full-fledged war that spreads into a major war between NATO and Russia,” he said. “We are working on that every day to avoid that.”

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Stoltenberg, a former prime minister of Norway, said in the interview that “there is no doubt that a full-fledged war is a possibility,” adding that it was important to avoid a conflict “that involves more countries in Europe and becomes a full-fledged war in Europe.”

Read more: AP