Computer networks stopped working entirely at three banks -- Shinhan, Nonghyup and Jeju -- around 2 p.m. Wednesday, Yonhap reported, citing the National Police Agency. Another financial institution, Woori Bank in Seoul, reported it was able to fend off a hacking attack about the same time.

The banks that were affected reported problems with a variety of systems, including Internet banking, ATMs and telecommunication services, and some branches stayed open late because of the slowdown, Yonhap said.