Washington, D.C. ? As North Korea fueled a multistage rocket Thursday for its threatened satellite launch, President Barack Obama promised a “stern” response and Japan vowed to press for an emergency session of the U.N. Security Council.

Senior U.S. defense officials said that trailers and vehicles carrying rocket propellant were in place at North Korea’s coastal launch site and that fueling had begun.

A U.S. counter-proliferation official said the fueling process could take “up to a few days.” But a senior U.S. intelligence official told The Associated Press that Pyongyang was on track for a projected Saturday launch.

The American officials spoke on condition of anonymity to discuss intelligence issues.