Barack Obama, the US president, has expressed concerned about cutbacks in European defence spending, telling NATO members in Brussels that "everyone has to be chipping in" to defend the continent's borders, sovereignty and territorial integrity.
Obama met Herman Van Rompuy, president of the European Council, and José Manuel Barroso, president of the European Commission, for trade talks although Russia's annexation of Crimea was high on the agenda and dominated the remarks following the first EU-US summit in more than two years.
Obama told a news conference on Wednesday: "There is a diminished level of defence spending in NATO countries.
"The situation in Ukraine reminds us that our freedom isn't free. We've got to be willing to pay for the assets, personnel and training required to make sure we have a credible NATO force and an effective deterrent force.
"This cannot just be a US and UK exercise. Everyone has to be chipping in."
In his remarks, Van Rompuy called Crimea's annexation "illegal" and a disgrace", but welcomed Russia's willingness to allow observers in to Crimea and its meeting with Ukraine's foreign minister.
"However if there is further escalation, we Europeans and Americans are ready to intensify sanctions," he said.
"With the understanding that sanctions are a means to an end. The goal is a negotiated solution, in respect of Ukraine's sovereignty and international law. We also stand by Georgia and Moldova."
At a later meeting, with NATO Secretary-General Anders Fogh Rasmussen, Obama referenced Article 5 of the bloc's treaty, which requires member states to come to the aid of any member state subject to an armed attack.
"The NATO commitments that we've made under Article 5 are something that are not just items on a piece of paper, but are critically important to all NATO members," he said.
"And we have to have the resources and preparation to make sure that every member of NATO feels confident in Article 5's effect.
"There will be a ministerial summit coming up at which I have asked the United States delegation to work cooperatively with the Secretary General’s office and evaluate all the additional steps that we might take in order to bolster that confidence among all NATO members."
On Thursday, as part of what the White House has called Obama's Spring 2014 trip, the president is meeting Pope Francis, the Italian president, Giorgio Napolitano, and the Italian prime minister, Matteo Renzi.
On Friday, Obama is due to meet King Abdullah of Saudi Arabia in Riyadh.