For North Koreans, the country's northern frontier long offered rare access to outside information, trade opportunities, and the best option for those seeking to flee.

But as the pandemic gripped the world in 2020, Kim Jong Un's regime embarked on a massive exercise to seal its borders with China and Russia, cutting off routes plied by smugglers and defectors.

Since then, Pyongyang has built hundreds of kilometers of new or upgraded border fences, walls and guard posts, commercial satellite imagery shows, enabling it to tighten the flow of information and goods into the country, keep foreign elements out and its people in.