Commuters move busily to catch subway trains in Seoul, in this file photo taken on Wednesday. (Yonhap)

Seoul Metro, the operator of subway lines 1-8, said Friday that 27 trains were delayed by more than 20 minutes on the previous day, marking the second day of a slowdown strike by its unionized workers.

With 3,189 subway trains in operation Thursday, their on-time rate stood at 99.1 percent, the operator said, noting that the current law defines a delayed train as one delayed by more than 20 minutes.

Compared with the first day of the unionized workers' "work-to-rule" protest, the number of delayed trains decreased by 98, and the on-time rate increased by 3.1 percentage points, it added.

The largest of Seoul Metro's three labor unions has threatened to go on a general strike on Dec. 6 unless their demands for wage and staff hiring increases are met.

The planned strike, along with a similar walkout threat from the unionized workers of KTX and regular train operator KORAIL, is expected to cause massive rail service disruptions nationwide early next month.

The Korean Railway Workers' Union has threatened to launch an indefinite general strike on Dec. 5, demanding higher pay and a larger workforce.

On Friday, 18 of 474 KORAIL-managed subway and above-ground trains that ran in the greater Seoul area from 5 a.m. to 9 a.m. were delayed by more than 20 minutes, up from 14 during the same hours the previous day, according to the company. (Yonhap)