BRUSSELS — Britain's remaining in the European Union is key for trans-Atlantic security and common efforts to fight violent extremism, the chief of the NATO alliance said Wednesday.
"It's up to the people of Great Britain to decide whether to remain or leave," NATO Secretary-General Jens Stoltenberg told The Associated Press. "But what I can do is tell you what matters for NATO. A strong U.K. in a strong Europe is good for the U.K., but it's also good for NATO."
Stoltenberg spoke one day before British voters are to cast ballots in a national referendum on EU membership.
"We are faced with so much uncertainty, so much unpredictability, with terrorist threats, with a more assertive Russia in the east," Stoltenberg said. "I believe that a more fragmented Europe will be something which will only add to the uncertainty which surrounds us."
NATO and the EU are currently stepping up efforts to cooperate on a range of issues, including against cyberattacks, widespread instability in the Middle East and North Africa and what Stoltenberg termed the "terrorist threats" posed by the Islamic State group and other extremist organizations.
"We need a strong Europe and a strong NATO together to address all these challenges, and the U.K. is key in making sure that that happens," Stoltenberg told the AP.
Like 21 other European nations, Britain currently belongs to both the U.S.-led NATO military alliance, which it helped found, and to the European Union economic bloc. Stoltenberg called Britain "a bridge" and a leader when it comes to promoting closer cooperation between Europe and North America, but also between NATO and the EU.
"This is important, because neither the European Union nor NATO has all the tools in the toolkit" to address present-day security challenges, Stoltenberg said.
Norway, Stoltenberg's home country, is a NATO member but voters there have twice rejected EU membership. Stoltenberg said for the British to opt for withdrawal from the EU could have consequences on a far greater scale.
"Since the United Kingdom is the biggest European NATO ally when it comes to forces, military capabilities, it really matters what the U.K. does," he said.
British government officials say their country is the sole European member of NATO that, like the United States, now meets the alliance's dual targets of spending 2 percent of national income in defense and committing 20 percent of that expenditure to buying new equipment.
With France, it is also one of only two European members of the alliance that possess nuclear weapons.
Following Russia's annexation of Crimea and what NATO views as Moscow's ongoing actions to destabilize Ukraine, Britain has also committed itself in 2017 to leading a ultrafast "spearhead" brigade of ground troops that was created as a response to the perceived Russian threat. It also will furnish the backbone of one of four new NATO multinational battalions designed to reinforce the defenses of Estonia, Latvia, Lithuania and Poland.
"The U.K. is Europe's pre-eminent military power and crucial to U.S.-European burden-sharing within NATO," said Ian Kearns, director of the European Leadership Network, a London-based think tank. If British voters on Thursday call for leaving the EU, he said, Britain could conceivably step up its role in NATO to demonstrate it remains a reliable ally.
"However, in the longer term, the big risk is that a political contagion effect will occur, strengthening euroskeptic movements around Europe and destroying the cohesion and solidarity necessary to the effective functioning not only of the EU but also of NATO," Kearns said. "The stakes in this referendum couldn't be higher."