The labour slowdown was launched by the Korean Railway Workers' Union to demand higher pay and a bigger workforce, with plans to begin an indefinite general strike early next month, Yonhap news agency reported.

As of 8 a.m., trains were being delayed by between 5 and 20 minutes on capital area subway lines 1, 3 and 4 and the Suin-Bundang, Gyeongui-Central and Seohae lines, according to the Korea Railroad Corp. (KORAIL).

All regular trains and KTX high-speed trains were operating normally.

A KORAIL official said an emergency task force was set up the previous day to maintain a prompt response system and minimise passengers' inconvenience.

Any violations of company rules or the law by the striking workers will be handled strictly, the official said.