Coronavirus cases tied to a Seoul detention center rose by 126 on Thursday, sending the total to over 900, while another inmate died of COVID-19 complications at a separate facility.

The justice ministry said 126 inmates at Dongbu Detention Center in southeastern Seoul tested positive during the prison's fourth mass testing Wednesday, bringing the total to 918, including 21 staff members. No additional workers tested positive this round.

At Seoul Detention Center in Uiwang, 25 kilometers south of the capital, one inmate in his 30s died Thursday morning of COVID-19 complications. The first such death was reported at Dongbu Detention Center earlier this week.

The government announced the start of a two-week period during which the highest-tier social distancing measures will be in place at prison facilities nationwide.

"The justice ministry tried to prevent the spread of COVID-19 inside correctional facilities, which are vulnerable to infection, but this situation arose from the limitations that are inherent to detention facilities and our failure to take antivirus steps in advance," Vice Justice Minister Lee Yong-gu said during a press briefing at the main government complex in Seoul.

"We are sorry to the nation. ... The justice ministry will strengthen antivirus measures and inspections at correctional facilities to ensure no further infections, and do everything we can to stabilize the situation," Lee said.

The Level 3 restrictions, in force until Jan. 13, ban in-person visits and cancel all work and classes for inmates. Meetings with lawyers will be limited and all staff members will work on a reduced schedule.

Wednesday's fourth round of testing involved 465 workers and 1,298 inmates at Dongbu Detention Center. Fourteen of the inmates have yet to receive their results.

Separately, 20 family members and one acquaintance have tested positive in connection with the prison, according to the health authorities.

The ministry has been looking at ways to transfer more inmates from Dongbu Detention Center to ease the population density there. The center is currently above capacity at 116.6 percent.

Hundreds of prisoners, including those who have tested positive for the coronavirus, have already been transferred to prison facilities nationwide.

The ministry is also considering releasing more inmates on parole, especially if they have an underlying disease or have demonstrated exemplary behavior.

To prevent further infections from asymptomatic patients, the ministry will conduct additional mass testing on the prison staff and all inmates, and set up more residential treatment centers within the prison compounds and in the surrounding neighborhoods to treat mild cases.

The ministry blamed the mass infections at Dongbu Detention Center on its layout and poor ventilation system, high population density and failure to anticipate the spread of the virus through asymptomatic patients.

The detention centers have added to South Korea's woes in combating the virus.

The country has reported around 1,000 new coronavirus cases a day since the middle of the month.

The capital area comprising Seoul, Incheon and Gyeonggi Province is currently under Level 2.5 restrictions, while the rest of the nation is mostly under Level 2. (Yonhap)