NATO Secretary General Jens Stoltenberg warned U.S. President-elect Donald Trump that "going it alone" is not an option for either Europe or the United States. Stoltenberg's comments came as Trump's victory raised fears of America pulling out of the defense of the High North and Baltic Sea regions.

Stoltenberg reportedly said that the West faced its greatest security challenge in a generation. He conceded that Trump had a point about the need for some members to make a bigger financial contribution. The U.S. currently accounts for almost 70 percent of NATO spending.

"It is all too easy to take the freedoms, security and prosperity we enjoy for granted. In these uncertain times we need strong American leadership, and we need Europeans to shoulder their fair share of the burden," the former Norwegian prime minister wrote in Britain's Observer newspaper. "Going it alone is not an option, either for Europe or for the United States. We face the greatest challenges to our security in a generation. This is no time to question the value of the partnership between Europe and the United States."

Trump has previously described NATO’s present mission and structure as "obsolete."

During his campaign, the Republican candidate suggested that as president he wouldn't help Baltic states in NATO against Russian aggression if they didn't pay their fair share. He also said that the U.S. bears a tremendous cost by being a member of NATO and the country has to be prepared to let European allies "defend themselves."

Stoltenberg also raised concerns over Russia's increasing military presence across its western border and that it should be treated as a genuine threat to which the West must respond. Trump has claimed that he will be able to build a strong relationship with Russian President Vladimir Putin.

“We have implemented the biggest reinforcement of our collective defence since the cold war. And the United States has significantly strengthened its commitment to European security, deploying a new armoured brigade to eastern Europe and delivering equipment and supplies to support future reinforcements if needed. This is deterrence, not aggression. We do not seek to provoke a conflict, but to prevent a conflict,” Stoltenberg said.