WASHINGTON (CNN) -- North Korea has begun fueling its long-range rocket, according to a senior U.S. military official.

A satellite image shows a rocket sitting on its launch pad in the northeast of the country.

The fueling signals that the country could be in the final stages of what North Korea has said will be the launch of a satellite into space as early as this weekend, the senior U.S. military official said Wednesday.

Other U.S. military officials said the top portion of the rocket was put on very recently, but satellite imagery shows a shroud over the stage preventing a direct view of what it looks like.

The officials said the payload appears to have a "bulbous" cover, which could indicate that there is a satellite loaded on it. Such a cover protects a satellite from damage in flight.

Although the sources did not know for sure what the payload is, they said there is no reason to doubt that it is a satellite, as indicated by North Korea.

Pyongyang has said it will launch the rocket between April 4 and April 8.

Pentagon officials worry less about the payload and more about the launch itself, saying that any kind of launch will give the North Koreans valuable information about improving their ballistic missile program.

The United States believes that the North Koreans have the technology to hit Alaska or Hawaii with a missile and that the country is working on advancing that technology so it could hit the west coast of the United States.

All About North Korea • Nuclear Proliferation • Kim Jong-il