NATO Secretary-General Jens Stoltenberg has said the West is facing some of its greatest security challenges and "going it alone is not an option."
Stoltenberg, in an article in Britain's The Observer newspaper on November 13, said that "we face the greatest challenges to our security in a generation," amid concerns about U.S. President-elect Donald Trump's views toward NATO.
"This is no time to question the partnership between Europe and the United States," Stoltenberg said.
Trump caused concern in March, during his election campaign, when he called NATO "obsolete" and said he would withhold U.S. support from alliance members unless they increased military spending and "fulfilled their obligations" to the United States.
Stoltenberg admitted in the article that European countries must increase their financial contributions to the security alliance, as the United States currently accounts for almost 70 percent of NATO spending.
But he noted that the only time the NATO clause of "an attack on one is an attack on all" was invoked was after the September 11, 2001, terrorist attacks against the United States.
He said some 1,000 European soldiers serving in Afghanistan had "paid the ultimate price" in what was "a direct response" to the 9/11 attacks.
Stoltenberg added that NATO had made possible the "integration of Europe" and ended the Cold War, saying that "European leaders have always understood that when it comes to security, going it alone is not an option."
"At the same time, American leaders have always recognized that they had profound strategic interest in a stable and secure Europe," he added.
The NATO chief also said the alliance still played a major role in fighting against terrorism and has responded to "a more assertive Russia."
"We have implemented the biggest reinforcement of our collective defense since the Cold War," he said. "And the United States has significantly strengthened its commitment to European security, deploying a new armored brigade to eastern Europe and delivering equipment and supplies to support future reinforcements if needed."
Moscow's relations with the West have plunged to levels of acrimony unseen since the end of the Cold War following Russia's military seizure of Ukraine's Crimean Peninsula in March 2014 and an ensuing war between Kyiv's forces and Russia-backed separatists.
Trump said in September that he "would have a very, very good relationship" with Russian President Vladimir Putin and also asserted that he would be better at negotiating with Putin than President Barack Obama has been.