South Korean protesters shout slogans and hold pictures of North Korean leader Kim Jong-il as they demonstrate against North Korea's rocket launch plans on Thursday in Seoul. (Lee Jin-man/Associated Press)

U.S. President Barack Obama warned of "a stern, united response" if North Korea proceeds with its planned rocket launch, amid Thursday reports that the country has started to fuel the device.

Obama met with South Korean President Lee Myung-bak ahead of the G20 summit in London on Thursday, where the two leaders agreed to work together on a "joint reaction" to the launch, which is expected to occur sometime between Saturday and next Wednesday.

North Korea has said it is sending a communications satellite into orbit as part of a multi-stage rocket, but the U.S., South Korea and Japan allege the plan is a cover for testing long-range missile technology.

Several media outlets reported that North Korea began fuelling the rocket on Thursday, citing unidentified senior U.S. military officials.

Japan chief cabinet secretary Takeo Kawamura said Thursday the U.S. and Japanese governments have not confirmed the fuelling has started. Experts have said that the rocket can be fired about three days after fuelling begins.

The South Korea Defence Ministry declined to comment.

Obama warned that a liftoff of the rocket would be a "provocative act" and reaffirmed that it would generate a UN Security Council response.

Regional powers have also begun to deploy ships to monitor the launch and Japan is preparing to intercept any debris that might fall if the launch goes wrong — moves that have prompted several threats of retaliation from Pyongyang.

"If Japan imprudently carries out an act of intercepting our peaceful satellite, our people's army will hand a thunderbolt of fire to not only interceptor means already deployed, but also key targets," said a general of the North Korean military in a report published by the North's official news agency.

With files from the Associated Press