In order to see a Korean Peninsula free of nuclear weapons, the U.S. must ask North Korea about its long-term objective, warned James Clapper, former American director of national intelligence.

A retired U.S. Air Force lieutenant general, Clapper weighed in on the state of Washington-Pyongyang relations in the wake of the historic June summit between U.S. President Donald Trump and North Korean leader Kim Jong Un.

"What failed to happen in the summit that occurred here in Singapore would have been a great opportunity for President Trump to get the answer to a very important question: 'What is it that you require in North Korea that you don't need nuclear weapons to feel secure?'" Clapper told CNBC's Nancy Hungerford at the annual Barclays Asia Forum in Singapore.

"It would be very useful to know the answer to that. Otherwise, we're ad hoc-ing it and taking one step in the blind at a time," said Clapper, who was appointed by former President Barack Obama in 2010 and left the role when Trump entered office.

The U.S. leader is considering meeting Kim for a second summit "as soon as possible," according to recent news reports. But Clapper expressed doubt about the results of such talks.

"Based on my own interactions with the North Koreans, I'm very doubtful they're ever going to agree to denuclearization," he said, adding that Washington had yet to define what exactly denuclearization means.