Unionized workers of the Seoul subway system continued their strike on Friday, with the operation rate of trains standing at around 80 percent of normal levels.

The two-day strike, which started at 9 a.m. on Thursday, is set to end at 6 p.m. Friday.

The union of Seoul Metro said earlier that this week’s strike is a warning and that there is the possibility of a large-scale strike after next Thursday if there is no progress in negotiations with management.

With the number of passengers likely to surge during morning rush hour on Friday, management plans to maintain the operation rate of subway trains at normal levels by mobilizing substitute workers.

Meanwhile, intra-city buses will extend operations from 7 a.m. to 10 a.m., one hour longer than usual.

On the first day of the strike on Thursday, trains were operating at 75-point-four percent of normal levels as of 6 p.m.