A historic summit between Donald Trump and Kim Jong Un — abruptly agreed to with little plan for how events would unfold — collapsed as the leaders talked past each other on their goals, phone calls weren't returned and rhetoric turned from hopeful to sour.

U.S. officials insist they are optimistic the meeting will happen, eventually. And North Korea's Vice Foreign Minister Kim Kye Gwan said Friday that his country remains willing to meet the U.S. any time.

But the failure to deliver the much-advertised summit set for June 12 in Singapore is a bitter lesson for the Trump administration. U.S. officials insist North Korea is responsible for the breakdown. Negotiators from Pyongyang failed to show up at a preparatory meeting in Singapore, according to a senior administration official, while efforts to coordinate plans over the phone brought only "dial tones," Secretary of State Mike Pompeo said.