President Obama on Wednesday expressed concern that NATO members were not spending enough on defense, saying it was important to ensure that all of the alliance's members were “chipping in.”

“If we've got collective defense, it means that everybody's got to chip in. And I have had some concerns about a diminished level of defense spending among some of our partners in NATO; not all, but many,” Obama warned during a joint press conference at the Council of the European Union in Brussels.

“The trend lines have been going down,” he continued, saying that while that was understandable during Europe's recent fiscal crisis, the crisis in Crimea should serve as a wake-up call.

“The situation in Ukraine reminds us that our freedom isn't free and we've got to be willing to pay for the assets, the personnel, the training that's required to make sure that we have a credible NATO force and an effective deterrent force,” said Obama.

“So one of the things that I think, medium and long term, we'll have to examine is whether everybody is chipping in,” he added. “And this can't just be a U.S. exercise or a British exercise or one country's efforts; everybody's going to have to make sure that they are engaged and involved.”

The president is slated to meet NATO Secretary General Anders Fogh Rasmussen on Wednesday as he continues his week-long visit to Europe.

The trip, though, has been over been overshadowed by Russia's seizure of Crimea from Ukraine. The U.S. and allies have struggled to find effective measures that will force Russian President Vladimir Putin to back down.

The U.S. and EU have instituted sanctions against individuals and have threatened to target key sectors of the Russian economy if the Kremlin does not de-escalate the situation.

Fears have grown that Russia may attempt another power grab in the region.

Obama said that Georgia and Ukraine were not on a path to NATO membership but that increased defense spending and maintaining a credible NATO force would “help build more confidence” among Russia’s neighbors.