Emmanuel Dunand | AFP | Getty Images
NATO Secretary General Jens Stoltenberg (R) and US Defence Minister James Mattis address the press during a NATO defense ministers' meeting at the NATO headquarters in Brussels on February 15, 2017.
Meeting the target will take longer for some countries than others, he admitted, and said he was confident all allies will meet the benchmark within a decade, as promised.
The contentious spending debate comes at a crucial time for NATO, as the partnership faces increased threats from conventional sources as well as cyber-terrorism and a "more assertive Russia", Stoltenberg said.
"We are seeing a more assertive Russia. We are seeing a Russia which has invested heavily in new military capabilities, which has tripled its spending on defense over the last years and, most important, which has been willing to use military force against neighbors in Georgia and Ukraine."
Stoltenberg said NATO is responding by increasing its presence in the East with new battle grounds and a bigger military presence.
"We have to be agile, we have to be prepared for the unforeseen, and that's exactly what we are doing."