WASHINGTON — As North Korea fueled a multistage rocket yesterday for its threatened satellite launch, President 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 Pyong- yang was on track for a projected launch tomorrow.

At the G-20 summit in London, Obama and South Korean President Lee Myung-bak issued a statement agreeing on "a stern, united response from the international community if North Korea launches a long-range rocket."

Department of Defense officials said they were not planning to shoot it down, but said the U.S. would increase diplomatic pressure on North Korea if it goes ahead, as expected. Pyong- yang would find itself further from what it wants from the outside world, including diplomatic recognition, trade and investment, and security guarantees.

"They need things," one U.S. official said.

Japan's ambassador to the U.N. said his nation will request an emergency session of the Security Council if North Korea proceeds with the launch. Yukio Takasu said he raised the possibility during closed-door council talks yesterday.

Takasu and other council diplomats say they anticipate a possible emergency session as early as this weekend.

Chinese President Hu Jintao urged Japan to handle the expected firing of a rocket by North Korea over Japanese territory calmly. He said Beijing is trying in various ways to dissuade Pyongyang from conducting the communications satellite launch that is widely believed to be a test for a long-range missile.

Quoting an unidentified North Korean general, the North Korean Central News Agency said Japan would be struck with a "thunderbolt of fire" if it attempts to intercept the multistage rocket.