ABOARD A US AIR FORCE AIRCRAFT ABOVE THE ATLANTIC OCEAN — US Secretary of Defense Ash Carter remains concerned about declining defense budgets in Europe, at a time he says NATO is facing new challenges .

"In general, our view is they're not investing enough," Carter said of US allies in Europe. "We'd like to see more. We understand the economic circumstances in general, but still in all, security is a very important thing to be investing in.

"I certainly will be continuing to argue that the Europeans should be making bigger investments."

That call is likely to be repeated over the next month when a trio of major European meetings – the annual G7 gathering, the NATO ministerial and the Council of Europe – gather key European allies together, a defense official said on background.

Carter made his comments June 5 to a group of reporters, just hours after the conclusion of a major review of US strategy as it relates to Russia.

The secretary pointed out that a few years ago, the discussion revolved around NATO's lack of a mission in a post-Cold War era. Now, he said, the alliance is faced with two distinct, but serious threats, in the form of Russian aggression and the Islamic State, commonly known as ISIS.

It's something he equated to chewing gum and walking at the same time, warning that it requires new commitments from the partners not just in dollars spent, but how they are investing them.

In particular, Carter called out the need to improve the information and intelligence sharing capabilities between partner nations.

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"[That's] something that's been discussed for a long time within NATO, and it's really time to make some progress on that," Carter said.

Carter's comments echo those of Gen. Denis Mercier, the incoming Supreme Allied Commander Transformation at NATO, who has called for the allied nations to invest in cloud technologies that would allow greater information sharing between partners.

And while pushing for NATO partners to invest in their future, Carter said the US also needs to take lessons learned from the last year of Russian actions – which has featured nontraditional warfare, cyber-attacks and a complex information war – and adjust its investments.

"Nobody has any illusions today that the right way to deal with what's happening in Russia is to turn the clock back ourselves 25 years, because military art has moved on," he said.

A second defense official added that the American military can be too reliant on old ways of intelligence gathering while ignoring the vast array of open source data now available thanks to social media and data processing.