The satellite images of the Norths Sohae complex on Wednesday and Thursday (Feb. 3 & 4) showed the trucks at fuel and oxidizer bunkers, the US-Korea Institute said on its website, 38 North.
"Contrary to recent reports, the presence of these tankers more likely indicates the filling of fuel/oxidizer tanks within the bunkers than the fuelling of the space launch vehicle", the group said on Friday.
"In the past, such activity has occurred 1-2 weeks prior to a launch event and would be consistent with North Koreas announced launch window of February 8 through 24," it added.
Pyongyang has announced it will launch a satellite-bearing rocket sometime between February 8-25, which is around the time of the birthday on February 16 of late leader Kim Jong-Il, father of Kim Jong-Un.
Citing an unidentified US Defence Department official, Japans Asahi Shimbun said Friday that US satellite data suggested North Korea may have already begun fuelling the rocket in the face of wide-spread international condemnation.
The North insists its space programme is purely scientific in nature, but the United States and allies, including South Korea, say its rocket launches are aimed at developing an inter-continental ballistic missile capable of striking the US mainland.
UN sanctions prohibit North Korea from any launch using ballistic missile technology.
The imminent launch would constitute another major violation of UN Security Council resolutions following Pyongyangs fourth nuclear test last month.