North Korea has strengthened its border patrol and dispatched agents to China to try to stem the flow of defections of its people, a South Korean official familiar with the issue said Thursday.

North Korean border guards along the country’s porous border with China have been put on high alert to arrest those who try to flee, the official said.

The official also said the North had sent its agents to China in cooperation with the Chinese authorities to try to find North Korean defectors and send them back to the North.

It is not the first time that the North has cracked down on defectors, but the latest crackdown came days after North Korea executed leader Kim Jong-un’s uncle, Jang Song-thaek, who had long been considered the North’s No. 2 man and Kim’s regent.

A constant stream of North Koreans has crossed the border into China’s northeastern areas to avoid chronic food shortages and harsh political oppression in recent decades.

Tens of thousands of North Korean defectors are believed to be hiding in China, hoping to travel to Thailand or other Southeast Asian countries before resettling in South Korea, home to more than 26,000 North Korean defectors.

Meanwhile, China does not recognize North Korean defectors as refugees and regularly repatriates those who are caught to the North, where they can face harsh punishment. (Yonhap News)