LONDON — The secretary general of NATO expressed confidence on Friday that President-elect Donald J. Trump would not back away from America’s longstanding commitment to European security, even as the alliance’s chief acknowledged that the region needed to shoulder a greater financial burden for its own defense.
Mr. Trump raised alarm during the election campaign when he questioned whether the United States would automatically defend NATO allies if they were attacked. Mr. Trump said American support would depend on the willingness of those countries to pay their fair share for military protection.
He has also called NATO “obsolete” and said that the alliance was failing to fight terrorism.
Those allies not willing to pay for American military protection, he warned, could receive a stark message: “Congratulations, you will be defending yourself.”
But in his first speech since the election last week, Jens Stoltenberg, the NATO secretary general, said that he looked forward to meeting Mr. Trump soon and that he was certain Mr. Trump would not waver on the United States’ role in the alliance.
“Europe needs America and America needs Europe,” Mr. Stoltenberg said. “I am absolutely confident that President Trump will maintain American leadership in the alliance and will maintain a strong commitment to European security. It is important for Europe, but it is also important for the United States.”
Mr. Stoltenberg, who is a former prime minister of Norway, said he would tell Mr. Trump that Europe was prepared to increase its financial commitment to the alliance, which he said was crucial at a time of increased instability, including a more assertive Russia; the threat of the Islamic State; turmoil in the Middle East; and cyberwarfare. “I will tell President-elect Trump that my main priority is that European allies increase defense spending,” Mr. Stoltenberg said.
Mr. Trump’s insistence that Europe pay more, Mr. Stoltenberg added, was hardly a new demand. He said he had heard the same message from other leading American officials, including President Obama and every member of Congress he had met.
The United States accounts for 70 percent of spending on the North Atlantic Treaty Organization, and Mr. Stoltenberg noted that other than the United States, only four NATO countries — Britain, Estonia, Greece and Poland — spent at least 2 percent of gross domestic product on defense, the alliance’s goal for its members. He stressed that if all NATO members matched that target, it would mean an additional $100 billion a year for the alliance.
In words that appeared calibrated for Mr. Trump, Mr. Stoltenberg stressed that the only time the Atlantic alliance had invoked its self-defense clause, which states that an attack on one member constitutes an attack on all members, was in support of the United States after the attacks of Sept. 11, 2001.
The alliance has played a significant role in coming to the aid of the United States, Mr. Stoltenberg said, taking a vital role in operations in Afghanistan starting in 2003, in the aftermath of 9/11. He added that thousands of European troops had served in Afghanistan and that more than 1,000 had died in that mission.
Noting that Friday was the centenary of the final day of the Battle of the Somme in World War I, Mr. Stoltenberg said that two world wars and the Cold War had shown that security in Europe relied on the United States and that European security was in America’s national interest.
Amid growing jitters among American allies that Mr. Trump could back away from security commitments that have defined international relations for decades, President Obama sought last week to reassure Europeans, saying in Athens that American support for NATO was robust and transcended whoever was in the White House.
Mr. Obama reiterated that point in Germany on Thursday, saying at a news conference with Chancellor Angela Merkel that he had been encouraged by what he called Mr. Trump’s “full commitment to NATO as the foundation for international security.”
However, calls for Europe to offset its dependence on American defense have been intensifying since Mr. Trump’s election. In an interview with Reuters this week, Roderich Kiesewetter, a spokesman on foreign policy for Ms. Merkel’s conservative bloc in the German Parliament, said that Europe needed to think about developing its own nuclear deterrent strategy, given the possibility of a retrenchment under Mr. Trump.
Mr. Kiesewetter said that Germany, the largest economy in the 28-member European Union, could play a central role in urging nuclear powers like Britain and France to take over from the United States in providing nuclear security guarantees for the rest of the region.