BRUSSELS - President Obama today warned NATO allies that in order for collective defense to work, all members need to "chip in" - saying the crisis in Ukraine is a reminder that "freedom isn't free."

"I have had some concerns about a diminished level of defense spending among some of our partners in NATO; not all, but many. The trend lines have been going down," he said at a news conference at the Council of the European Union. "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."

Calling NATO the "cornerstone" of U.S. national security, Obama had warned Tuesday that Russia would face a coordinated response if it encroaches on NATO countries.

"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. And I think that will help build more confidence among some of those border states," he said today.

"We have to make sure that we have put together very real contingency plans for every one of these members, including those who came in out of Central and Eastern Europe," he said.