American missionary Kenneth Bae speaks to reporters at Pyongyang Friendship Hospital in Pyongyang, Jan. 20, 2014. Kim Kwang Hyon/AP Photo

BEIJING - Imprisoned American Kenneth Bae was sent to a North Korean labor camp in part due to the regime's anger over supposed American B-52 bomber flight drills around the Korean Peninsula last week, officials told ABC News.

North Korean officials broke the news by telling Donald Gregg, a former ambassador to South Korea and an ABC News consultant who was on a rare visit to Pyongyang, the North Korean capital.

"Rhee Young-Ho, a first vice minister, said that the memory of the B29 air raids are in the [North Korean] DNA," Gregg told ABC News today during a stopover at the Beijing International Airport while en route back to the U.S. "[Rhee said] to have the B52s which are nuclear capable fly over their air space is seen as a really terrible, terrible threat."

The Pentagon has acknowledged the "rotational presence" of bombers in the region, but would not confirm the details of the mission that angered the North Koreans.

Gregg was traveling with a group from the Pacific Century Institute on the invitation of the North Korean Foreign Ministry to discuss economic development. The five day visit to Pyongyang came at the same time as North Korea rescinded an invitation for a senior U.S. envoy to visit the reclusive nation to seek Bae's release.

"The question of Kenneth Bae immediately came up. That's not why we were there. It was just coincidental. I immediately said you ought to return him immediately," Gregg told ABC News.

Bae, a 45-year-old Korean-American, has been held in North Korea since November 2012 and is the longest-serving American prisoner in North Korea since the height of the Korean War. In April 2013 Bae was sentenced to 15 years of hard labor for allegedly trying to overthrow the state.

North Korean officials also told Gregg that they are not optimistic about improved relations with the U.S. because they feel under pressure.

"Rhee said to my disappointment, 'We really don't think much progress is going to be possible with President Obama. But we're prepared to outwait him. We are very happy with our new leader [Kim Jong-un].'"

During the talks Gregg urged the foreign ministry officials to not give up on the prospect of dialogue between the two countries.

"I expressed the hope that we could get to know [Kim Jong-Un] better, " Gregg said. "And that he could get to know us better. I also urged them not to give up on President Obama."

One silver lining Gregg detected was that the North Koreans were very interested in improving their economy, which was the main focus of their discussions.

"They're interested in developing their country economically," said Gregg. "And they're very interested in having help in doing that."

The visit was also Gregg's first visit to the North Korean capital in 12 years and he told ABC News that the city had changed a great deal since he was last there.

"The city is blossoming. Everybody has cell phones. New buildings have been built. Flashy new restaurants are thriving and so forth," he said.

Gregg told ABC News that the highlight of the trip was arranging for a member of his group, former Californian congressman Pete McCloskey, a Korean War Veteran, to meet an 81-year-old, retired 3-star North Korean general.

"The North Korean general had been in some of the same battles that Pete McCloskey had been. McCloskey won the Navy Cross and the Silver Star for his efforts. And the two met. It was very moving."