(CNN) - North Korean leader Kim Jong Un’s execution of his uncle was “an ominous sign” of instability and danger, U.S. Secretary of State John Kerry said in an interview broadcast Sunday.
Kerry also weighed in on reports about former FBI agent Robert Levinson, who went missing in Iran seven years ago. The United States has not stopped efforts to get him released, Kerry told ABC's "This Week."
The secretary of state is on an overseas trip that's taken him to Jerusalem, Ramallah, Vietnam, and will carry him through Manila and Tacloban later this week.
'Spontaneous' and 'erratic'
The killing of Kim's uncle – the latest execution of many in recent months, Kerry said - shows the degree to which Kim is "reckless" and "insecure."
"And most importantly, it underscores the importance for all of us of finding a way forward with North Korea in order to denuclearize the peninsula," Kerry said, adding that the U.S. has made "progress" with China in the effort.
Jang Song Thaek, Kim's uncle by marriage, was, until recently, regarded as the second-most powerful figure in North Korea. He was considered to be the regent who secured Kim's assumption of power after the 2011 death of his father, Kim Jong Il.
The country remains "opaque," but the insights the U.S. government has about Kim indicate the young leader is "spontaneous, erratic, (and) still worried about his place in the power structure," Kerry said. He stressed that China, Russia, Japan, South Korea and the United States all need to "stay on the same page" about the urgency of removing North Korea as a nuclear threat.
"To have a nuclear weapon potentially in the hands of somebody like Kim Jong Un just becomes even more unacceptable," he said.
On Levinson, Kerry says he has 'personally raised the issue'
Meanwhile, Kerry hit back against suggestions that the United States has abandoned Levinson. The Associated Press and the Washington Post reported Thursday that he was in fact working for the CIA in Iran, not conducting private business as officials have previously claimed.
"There hasn’t been progress in the sense that we don’t have him back. But to suggest that we’ve abandoned him or anybody...is simply incorrect and not helpful," Kerry said. "The fact is that I have personally raised the issue, not only at the highest level that I have been involved with, but also through other intermediaries."
Both the State Department and Levinson's family have long denied he was working for the U.S. government when he disappeared on a trip to Iran in 2007. But Thursday's reports from the Washington Post and the AP said Levinson was on a CIA mission to dig up information.
A source involved in the matter told CNN there's proof that Levinson worked for the CIA undercover and under contract while also working as a private investigator.
Kerry said the United States is still looking for proof that Levinson is alive. A “number of different channels” are “being worked aggressively,” he said.
In 2011, the State Department said new evidence suggested that Levinson, who has diabetes and high blood pressure, was alive and being held somewhere in southwest Asia.
This year, a source with knowledge of the investigation told CNN, "We have every reason to believe that he's alive and that the Iranians control his fate."
Asked whether he believes the Iranian government is responsible for his disappearance, Kerry said he thinks the government "has the ability to help us here, and we hope they will."
During a September interview with CNN's Christiane Amanpour, Iranian President Hassan Rouhani was asked what he could tell Levinson's family.
"We don't know where he is, who he is," Rouhani said. "He is an American who has disappeared. We have no news of him."
– Ashley Killough, Jethro Mullen, Tom Watkins, Susan Candiotti, and Catherine E. Shoichet contributed to this report.