WASHINGTON - North Korea has started fueling a rocket ahead of a planned satellite launch that could come as early as this weekend, CNN television reported on Wednesday, citing an unnamed US military official.

Fueling would confirm the regime is entering the final preparations for the launch which it has announced will occur during an April 4-8 window.

A senior defense official, speaking on condition of anonymity, told AFP on Wednesday afternoon there was no clear sign yet that fueling had begun.

US Secretary of State Hillary Clinton warned during a visit to Tokyo that the North Korean launch would have consequences at the UN Security Council, saying: "It is an unfortunate example of provocation by the North Koreans."

Pyongyang threatened on Wednesday to shoot down any US spy planes if they violate its airspace to monitor the imminent launch, in a statement carried by state radio.

Recent commercial satellite pictures suggest preparations for a launch, with the removal of tarpaulins that had covered the shape of the rocket's tip, according to weapons experts.

The rounded shape of the rocket nose indicated it was a satellite, as North Korea has announced, and not a warhead, two arms analysts said.

A warhead requires a narrower outline, resembling a sharpened pencil, to allow it to survive re-entry into Earth's atmosphere, said Bruce Bennett, a senior defense analyst at the Los Angeles-based Rand Corporation.

"Unless the front end of the missile as we're seeing it now is just a nosecone covering the warhead - which is a possibility - unless it's that, then it doesn't look very much like a warhead," Bennett told AFP.

"A warhead would not be as rounded," he said.

A satellite can have a simpler design and "a relatively blunt nose" as it is headed into space, he said.

The latest commercial photos were "pretty grainy" but "it doesn't look like a warhead re-entry vehicle," said David Albright of the Washington-based Institute for Science and International Security. --AFP