The lights are on at North Korea's Ryugyong Hotel, the world's tallest unoccupied building.

Three rooms at the very top of the so-called 'hotel of doom', which is still not open and has seen a number of halts in construction over 29 years, were seen to be lit up in a grainy video.

An anonymous source told NK News it might have been due to a visit to Pyongyang by Naguib Sawiris, the chief executive of Orascom. The Egyptian telecommunications company previously carried out work on the building that included adding glass panels to the outside and a phone mast.

Peter Ward, a researcher based in South Korea, also told the site after the release of the October video: "I’m very surprised to see the top of the Ryugyong lit.

"Maybe Orascom did more than give the building a fresh plate of glass.

"The fact they have added an aircraft warning light is interesting, if slightly puzzling."

After 25 years of construction a glimpse inside North Korea's still unfinished 'Hotel Of Doom' Show all 4 1 /4 After 25 years of construction a glimpse inside North Korea's still unfinished 'Hotel Of Doom' After 25 years of construction a glimpse inside North Korea's still unfinished 'Hotel Of Doom' AN9574830In-this-Sept.-23-2.jpg Rare images have emerged of the pyramid shaped 105 storey building in Pyongyang which was labelled by Esquire magazine in 2008 the "worst building in the history of mankind". AP After 25 years of construction a glimpse inside North Korea's still unfinished 'Hotel Of Doom' AN9574975In-this-Sept.-23-2.jpg The pyramid-shaped 105-story Ryugyong Hotel stands in Pyongyang, North Korea. AP After 25 years of construction a glimpse inside North Korea's still unfinished 'Hotel Of Doom' AN9575005In-this-Sept.-23-2.jpg Visitors tour the top floor of the 105-story Ryugyong Hotel in Pyongyang After 25 years of construction a glimpse inside North Korea's still unfinished 'Hotel Of Doom' AN9575001In-this-Sept.-23-2.jpg A visitor, center left, stands on the ground floor of the 105-story Ryugyong Hotel in Pyongyang, North Korea. AP

Work began on the imposing structure in 1987, but construction was stalled for 16 years after the money for the project ran out.

The 330m tower has a total of 3.9 million ft² floor space and should have initially opened in 1989.