SEOUL — North Korea’s cyber army, long considered a midlevel security threat, is quietly morphing into one of the world’s most sophisticated and dangerous hacking machines.

Over the past 18 months, the nation’s fingerprints have appeared in an increasing number of cyberattacks, the skill level of its hackers has rapidly improved and their targets have become more worrisome, a Wall Street Journal examination of the program reveals. As recently as March, suspected North Korean hackers appear to have infiltrated Turkish banks and invaded computer systems in the run-up to the Winter Olympics, cybersecurity researchers say.

For years, cybersecurity experts viewed North Korea as a second-rate hacking force whose attacks were disruptive but reasonably easy to decode. Researchers rated its operational skills well behind countries such as Russia, Israel and the U.S.

Those days appear to be over, with Pyongyang flashing levels of originality in its coding and techniques that have surprised researchers. It also has shown a willingness to go after targets such as central banks and point-of-sale systems. As North Korea prepares for possible negotiations with Washington aimed at freezing its nuclear program, its hacking capabilities could help it generate money to compensate for economic sanctions or to threaten foreign financial institutions.

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North Korean hackers now among the world’s most dangerous