In the wake of the recent defection of a high-ranking North Korean diplomat to South Korea, concerns have grown that Pyongyang may be considering terrorist attacks on South Korean nationals in China and East Asian countries.

Last week, Seoul said Thae Yong-ho, a minister at the North Korean embassy in London, defected to the South with his family in the latest development that shows even elite officials are becoming disillusioned with the world's most closed and repressive country. Thae is believed to come from the top echelons of North Korean society and was trusted by the regime.

North Korean leader Kim Jong-un has reportedly ordered the dispatch of agents and even sought alliances with organized gangs in those regions to launch a terror attack against or kidnap South Korean citizens in retaliation for a series of high-profile defections of North Koreans.

According to a source on Sunday, the Thae Yong-ho case seems to have added fuel to the fire. Various signs point to moves to harm South Korean nationals having increased since Seoul confirmed last week that the diplomat has arrived in the country.

Accordingly, safety alerts have issued by Seoul, particularly among South Koreans living in China and those traveling to North Korea-China border areas.

"I can't give out the full details but the North Korean leadership has been infuriated over the series of recent incidents," the source said.

"For North Korea, provocative acts that won't leave any trace will be the easiest, like hiring local gangsters to commit a terrorist attack," the source claimed, adding that border areas with lax security are prone to such covert operations.

In April, a South Korean missionary was killed in Changbai in Jilin Province, China, which was believed to be committed by three North Korean secret agents.

Amid heightened security risks, the number of South Korean tourists to Mount Paekdu, the highest mountain on the Korean Peninsula situated on North Korea’s border with China, has fallen to half in 2016 of what it was a year ago.

According to travel agencies in China on Sunday, the number of South Korean tourists to the mountain region used to be around 2,000-3,000 a day, but this year the number dropped to less than half.

Normally, more than 15,000 Chinese and foreign tourists used to visit the mountain daily, and South Koreans made up nearly 80 percent of the foreign travelers.

"In the runup to the peak season of July and August we used to receive inquiries from April. But this year, the number trickled to less than half," said one staff member at a travel agency. "The dramatic drop was largely due to the rumors that North Korea was preparing a terror attack on South Koreans."

A worker at another travel company said, "When our South Korean partner companies ask if it is safe to travel here, we feel helpless because we can't guarantee absolute safety for the travelers."

Since a spate of high-profile defections by North Koreans and the subsequent deterioration of inter-Korean relations, Seoul repeatedly issued travel warnings to travel agencies and individuals against visiting the mountain.

Seoul's consulate in Shenyang has issued such warnings five times to South Koreans living in China. (Yonhap)