Trump said Wednesday that his administration is negotiating for the detainees' release and thinks "there's a good chance of doing it." The Americans -- Kim Hak-song, Tony Kim and Kim Dong-chul -- have been accused of espionage or "hostile acts."

Richardson, who played important roles in past dealings with Pyongyang, also said he thinks the Trump-Kim meeting could take place in Geneva or Russia.

"One, I'm betting on Geneva. Second bet is one of the Russian islands," he said.

On Trump's new national security adviser, John Bolton, the former governor noted his personal ties to him as a fellow former ambassador to the U.N.

"Everybody is worried that John Bolton is going to bomb (North Korea). Relax," Richardson said. "His first job is that this summit is a success as a staff member."

Bolton recently argued for a preventive strike on North Korea, although he also said it is not his favored option. (Yonhap)