US satellite data suggests North Korea may have already begun fuelling a rocket it plans to launch this month in the face of international opposition, a Japanese newspaper reported Friday, citing a US defence official.

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 current supremo Kim Jong-Un.

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.

North Korea "will finish preparations for the launch as soon as the next several days", the unidentified US Defense Department official told the Asahi Shimbun.

Since Thursday, satellite images have shown increased movement of people and equipment around the launch pad and a fuel storehouse at North Korea's Sohae satellite launch complex in the country's northwest, the official said, according to the Washington-datelined report.

The United States judged that fuelling appears to have started as it has been monitoring Pyongyang's movements via military intelligence satellites which can analyse objects as small as 30 centimetres (12 inches), the official was quoted as saying.

Given the difficulty in stopping the process once fuelling begins, preparation work normally finishes within several days of that, the official told the Asahi.

- Flights re-routed -

UN sanctions prohibit North Korea from any use of ballistic missile technology, and the imminent launch would amount to another major violation of UN Security Council resolutions following Pyongyang's fourth nuclear test last month

In its formal notification sent to UN agencies, North Korea provided flight coordinates similar to its last successful launch of a three-stage Unha-3 rocket in December 2012.

The separated first stage was predicted to fall in the Yellow Sea off the west coast of South Korea, followed by a second stage splashdown in the Philippine Sea.

In order to avoid any possible collision, Japanese and South Korean airlines will be rerouting a number of flights during the launch window period.

Japan's two biggest airlines -- All Nippon Airways and Japan Airlines -- said they would be diverting planes that fly over waters off the Philippines.

The change will affect three ANA flights -- from Tokyo's Haneda airport to Manila, from Manila to Tokyo's Narita International Airport, and from Jakarta in Indonesia to Narita, the airline said.

Two JAL flights -- one from Jakarta to Narita and another from Narita to Manila -- will also be rerouted.

South Korea's Transport Ministry said a total of 39 flights to and from the southern resort island of Jeju would be re-routed, involving the country's two main carriers Korean Air and Asiana Airlines, as well some Chinese carriers.

Another 36 flights from Korean Air and Asiana will adjust routes to avoid waters off the Philippines, the ministry said, adding that the changes would cause an additional average flight time of six minutes.