North Korea has built a cover around its rocket-launching platform possibly to evade from surveillance by spy satellites.
On Sunday, the US satellite images indicated the steel platform at Sohae Satellite Launching Station was completely covered.
The US diplomatic sources were quoted as saying by various media on Saturday that the previous 50-meter-tall gantry at the site has been extended to 60 meters in height. It is a clear indication that North Korea is accelerating preparations for a missile launch, the sources added.
The latest progress at the station in Tongchang-ri, North Pyongan Province near the China border, was detected after Seoul said that the North Korea recently built a 17-meter extension to a 50-meter launch pad.
The roof is expected to be end in mid-August.
Seoul expects that any kind of launch of North Korea on the October 10 Workers' Party anniversary would be used as a provocation against the South.
South Korean Defense Minister Kim Min-seok said Monday, it would be difficult to find out whether or not a missile is being assembled under the rooftop.
"(But) The South Korean and U.S. intelligence authorities will detect the signs well enough through various intelligence tools if the North pushes for a long-range missile provocation such as the one the country did in the past under the cover of a satellite launch," Kim added.