"I think it's incredibly important," State Department spokesman Matthew Miller told reporters of the NATO spending estimates unveiled earlier in Brussels.

NATO Secretary-General Jens Stoltenberg said that 18 of 31 members were meeting the alliance's goal of spending two percent of gross domestic product on defense -- up from 11 last year and just three in 2014.

"We continue to see progress from other countries towards meeting those targets, and we will urge those who have not yet met them to continue to take steps to do so," Miller said.

"NATO is an alliance that the the American people derive tremendous benefit from (and) provides tremendous security to the United States," Miller said.