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The U.S. and its allies Thursday stepped up pressure on North Korea to scrap its planned satellite launch, with President Barack Obama agreeing with the South Korean president to respond sternly if Pyongyang fires a long-range rocket.

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

“They need things,” said one U.S. official, who spoke on the condition of anonymity.

Fueling of the North Korean rocket has recently begun, and trailers and vehicles with propellant have been observed at the launch site, U.S. officials said.

At the G-20 summit in London, Obama met with President Lee Myung Bak of South Korea on Thursday and discussed North Korea at length, according to a senior administration official.

According to a statement released by the White House, the two men promised to “peacefully and verifiably” eliminate North Korean nuclear weapons and related programs.

Concern about the imminent launch is based on a belief that it is intended to serve as an intercontinental ballistic missile test by the nuclear-armed rogue nation. Western nations believe the satellite launch would be illegal under UN resolutions restricting North Korea’s missile programs.

North Korea heightened its rhetoric Thursday, saying it would retaliate against any attempts to destroy its rocket and also threatening to shoot down U.S. surveillance aircraft. U.S. officials did not take the threat seriously.

Defense Secretary Robert Gates spoke to his counterparts in Japan and South Korea late Wednesday, saying the U.S. will live up to its commitments to defend its allies.

North Korea has insisted it will use the launch to put a satellite into orbit. And U.S. officials said they do not believe the rocket poses a direct danger to America.

Defense officials said the legality of shooting down a rocket carrying a satellite was questionable, even if the North Korean launch was illegal under UN resolutions.

Officials expect a launch to come as early as Saturday, although the window for the rocket to be fired extends through Wednesday.

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