A firefighter walks inside Dongnimmun Station on Friday. (Yonhap)

A fire broke out at 6:24 a.m. in a Seoul subway tunnel on Line No. 3, with smoke suspending train operations between Yaksu Station to Gupabal Station for nearly two hours, according to Seoul Transportation Corp. on Friday.

The fire broke out in a tunnel between Muakjae Station and Dongnimmun Station, authorities said. Seoul Metro resumed operations between Yaksu Station and Gupabal Station at 8:12 a.m. Trains did not run for two hours in either direction as smoke plumed from the subway tunnel.

No casualties have been reported.

The cause of the fire, which was extinguished at 7:36 a.m., has yet to be disclosed.

The transport authorities said the morning rush hour operation of the subway and bus systems will be extended by an hour until 10 a.m.

Firefighters walks along a tunnel between Muakjae Station and Dongnimmun Station on Friday. (Yonhap)

The disruption comes after a series of subway system malfunctions caused inconvenience to commuters in Seoul this week as most of the country was under a cold snap.

On Thursday afternoon, train services on Line No. 7 from Cheongdam Station to Taereung Station were shut down for two hours due to a power outage.

It was preceded by another power outage on Gimpo Goldline on Wednesday that caused subway delays, as well as a door malfunction at Suraksan Station on Line No. 7 on Monday.

Commuters wait for a bus near Muakjae Station on Friday (Yonhap)