Three people were killed and more than 100 injured in a fire that swept through an apartment building north of Seoul Saturday, firefighters said.

The death toll from the blaze at Euijeongbu City could rise as seven of the injured were in critical condition, Yonhap news agency said.

"Three people were confirmed dead and 101 others were injured", city mayor Ahn Byeong-Yong told journalists.

A total of 175 people lived in the 10-storey block, he said.

The fire apparently started at a post box on the ground floor and spread quickly, sparking speculation it might be an arson attack, Yonhap said.

Many residents were unable to evacuate as the building's front entrance was blocked by smoke and flames.

A total of 25 fire engines, four helicopters and and 145 firefighters were mobilised to bring the fire under control.

"We'll keep all possibilities open as we investigate" to determine the cause of the blaze, a senior police officer said.