
"This is progress. But I will call on allies to keep the momentum, and to do more. Because we need to match our defense spending with the security challenges we face," Stoltenberg said.
His remarks came at the beginning of a two-day NATO defense ministerial, which U.S. Defense Secretary Ash Carter is attending.
Stoltenberg's remarks also come amid increased American public scrutiny on NATO. Presumptive Republican presidential nominee Donald Trump has repeatedly attacked the alliance, and questioned its value to the U.S.
The U.S. currently pays for more than 70 percent of NATO's total defense spending, and only five of its 28 members meet a requirement to spend 2 percent of their GDP on defense.
Trump has also criticized NATO for not doing enough to counter a growing terrorist threat in Europe, and deal with the refugee crisis.
Stoltenberg said NATO members will "look at the broader arc of instability beyond our borders" over the next two days and consider what more NATO can and should do to "project stability."
A major biennial NATO summit is planned next month in Warsaw.