The United States' alliance with the European Union is "unbreakable", U.S. Secretary of State John Kerry said after meeting EU foreign ministers in Brussels on Monday.

"The U.S.-EU partnership is strong," he told a news conference. "Strong today, will remain strong in the future. It is ensuring and it is unbreakable."

Recalling a weekend visit to a World War Two U.S. military cemetery in Luxembourg and the U.S. contribution to liberating Europe, he said cooperation was as intense now as it has ever been and that all sides, including British Foreign Secretary Boris Johnson, were determined to have as smooth a transition in the relationships as possible as Britain moves to leave the EU.