The number of visa overstayers detained by the Immigration Bureau for more than six months rose in 2017, the Justice Ministry announced Monday.

Of the 1,386 people being kept at the bureau's 17 detention centers around the country, 510, or about 37 percent, had been held for more than six months as December 2017. This amounts to a 9 percent rise from 2016.

The surge appears linked to the ministry's crackdown on "provisional releases," which are given to people with conditional deportation orders. The releases allow overstayers to reside outside detention centers temporarily. Those granted provisional release are prohibited from working and can only stay within areas designated by the ministry.