Passengers in the capital region face potential disruptions as unionized railroad workers begin a partial work-to-rule protest Monday, warning of an indefinite strike next month.

The work-to-rule protest by the Korean Railroad Workers' Union demanding a wage increase and the hiring of more staff caused slowdowns in subway services in the capital area during peak hours for commuters on Monday morning.

The labor action comes after the unionized workers at the state-run Korea Railroad Corp., also known as Korail, demand an increase of 2.5 percent in their basic salary and the company to hire more workers as they claim the company is severely short-staffed.

The union said in a press release that the Finance Ministry is pushing to cut the number of Korail employees by 1,566 positions despite the staffing shortage, which would worsen workloads, conditions and safety for both workers and passengers. The national railway operator, however, said it has "no plans yet" to downsize.

The union also demands better working conditions, such as shifting to a four-team, two-shift system to ensure that railway workers do not work the night shift for two days in a row, and implementing a fair promotion system.

Unionized workers plan to launch an indefinite general strike early next month if they fail to shake hands over their demands with the company, which could be the first strike since September last year. They will announce their future course of action during a press conference slated for Thursday.

The work-to-rule protest, in which productivity and efficiency are slowed down while workers strictly comply with safety regulations and laws, led to some 140 trains in the metropolitan area to run 10 minutes behind schedule as of Monday at 9 a.m., according to Korail. Also, approximately 120 trains were delayed by more than 20 minutes, with passengers experiencing extended wait times.

Korail operates the following lines in the metropolitan area: Line No. 1 from Yeoncheon to Kwangwoon University to Guro Station, and also from Guro to Incheon, Sinchang and Gwangmyeong Station; Line No. 3 from Daehwa to Samsong Station; Line No. 4 from Seonbawi to Geumjeong to Oido Station; the Suin-Bundang Line; the Gyeongchun Line; the Gyeongui-Jungang Line and the Seohae Line.

The disruptions affected key commuter subway lines in the capital area, such as lines No. 1, 3 and 4 -- lines that carry a large number of passengers and connect key areas in Seoul and the surrounding region, as well as the Suin-Bundang Line -- which connects southern Seoul and Incheon -- and the Gyeongui-Jungang and Seohae Lines, officials added. All regular trains and high-speed KTX trains were operating normally.

Korail explained that a number of workers had deliberately slowed down operations as part of the work-to-rule protest by saying they would visit the bathroom before starting their shifts while still adhering to workplace guidelines.

To cope with the disruptions, Korail notified passengers of cancellations of train services and changes in train destinations.

During morning peak hours at Seoul's Yongsan Station -- through which run lines No. 1 and 4 along with the Gyeongui-Jungang Line -- an announcement was made every 10 minutes on the metro platform, advising passengers to consider using other transportation options due to the delays, according to reports. Reports added that some overcrowded subways could not accept additional passengers to board.

While service delays will not likely impact passengers during the daytime, Korail noted that the delays in train services could disrupt operations during evening rush hours.

Meanwhile, the national railway operator said it had set up an emergency task force the previous day to minimize passengers' inconvenience and further problems. Officials added that they would take "stern action" against any violations of company rules or the law by protesting union workers.

In September last year, the union held its first collective action in about four years by launching a four-day general strike, asking for improvements in working conditions and allowing KTX train services to access Suseo Station in southern Seoul.