North Korean leader Kim Jong-un's decision not to visit Russia shows that the young leader is concerned about his control of the communist nation and feels less secure than often assumed, a U.S. expert said Tuesday.

Russia announced last week that Kim called off a planned visit to Moscow due to internal affairs. Moscow had said for months that Kim had accepted its invitation to attend celebrations marking the 70th anniversary of the end of World War II.

"He's concerned about control, positive control over the country. If you feel secure in your position, you don't need to execute lots of senior officials, right? But he's executing lots of senior officials," said Van Jackson, a North Korea expert at the Center for a New American Security.

"If you feel secure in your position, you can leave your country without needing to tend to domestic affairs. So, it's just two data points but they point in the same direction. At least, he's less secure than we often assume," he said during a discussion hosted by 38 North, a Johns Hopkins University program specializing in North Korea issues.

Kim took over control of the North after his father and later leader, Kim Jong-il, died in late 2011. Since then, he has purged a series of senior officials, including his powerful uncle, Jang Song-thaek, as he consolidated his grip on power.

Jackson, who served as a strategy adviser and a senior country director for the Pentagon from 2009 to 2014, also expressed concern about North Korea's massive arsenal of missiles, saying South Korea's defense against the North's missile threats is not enough.

"Current missile defense posture on the peninsula is not great.

It's woefully outgunned relative to what North Korea has and can do. Over a thousand missiles of various shapes and sizes from short range cruise missiles to ICBM capability, even if it's rudimentary," he said.

Some of the North's missiles are even said to be capable of carrying nuclear payloads, he said.

"Whether that's true or not, eventually they could all be capable of that," he said.

Jackson stressed that South Korea needs a "deep penetrating radar coupled with layers of interceptors," which he said gives "multiple tries, as many tries as possible at the same missile that's coming in." Deploying the THAAD (Terminal High Altitude Area

Defense) system would be of great help, he said.

The possibility of the U.S. deploying a THAAD battery to South Korea is one of the most sensitive defense and political issues in Seoul as China has expressed strong opposition to such a deployment, which it regards as a threat to its security.

Jackson rejected Chinese concerns.

"The biggest concern that I heard from China was that the radar associated with THAAD would be able to see inside China ... But it can't see inside the Chinese bedroom. What can the radar see? It can see emitters off of incoming missiles," he said.

"So unless China is in the business of firing missiles at South Korea, the radar argument is a weak lead. It's spurious," he said. (Yonhap)