The sequence has been practiced for months as the Pentagon develops the procedures necessary to counter such an attack, Marine Corps General James Cartwright told the Senate defense appropriations subcommittee.

A Boeing Co. ground-based anti-missile system has not yet been declared fully ready for operations though it would work in an emergency, Cartwright said.

"As you can imagine, getting the president, the secretary, the regional combatant commander into a conversation and a conference in a three to four minute timeframe is going to be challenging," Cartwright said.

His comments came as South Korea reacted to North Korea's claim that it removed spent fuel rods from a nuclear reactor, a step that could provide plutonium for a bomb.

South Korea expressed "grave concern" over today's announcement by the North's official Korea Central News Agency that 8,000 fuel rods were taken from a reactor in Yongbyon, 90 kilometers (56 miles) north of Pyongyang, Foreign Ministry spokesman Lee Kyu Hyung said in a statement.

"Such actions by North Korea aggravate efforts to resume talks aimed at denuclearizing the Korean peninsula," Lee said. "We strongly urge North Korea to immediately cease these actions, which may negatively effect such efforts, and return to talks without delay."

South Korea joined the U.S., China, Japan and Russia in trying to persuade North Korea to dismantle its nuclear arsenal. The North suspended its participation in the talks in February after saying it possessed nuclear weapons.

In Washington, State Department State Department spokesman Richard Boucher played down North Korea's claim regarding the fuel rods, saying "they've made similar statements in the past about this."

Boucher sidestepped questions about a report that the U.S. ambassador to Japan, Thomas Schieffer, told Japanese officials today that North Korea had "taken some preparatory steps" toward conducting a test of a nuclear device.

"I wouldn't quite read as much into his statements as you do," Boucher said in response to a question at a briefing.

President George W. Bush's spokesman, Scott McClellan, in a separate briefing, warned North Korea not to engage in "provocative action," language he's used previously in addressing reports of a possible nuclear weapons test.

The U.S. has refused to confirm a report in the New York Times last week that spy satellites showed construction work on a possible test site.

Boucher and McClellan called on North Korea to rejoin the six nation talks.

The U.S. is counting on China to use its "considerable influence" to bring North Korea back to the negotiations, McClellan said.

Chinese Foreign Ministry spokesman Liu Jianchao said during a briefing yesterday in Beijing that his country opposes any further steps on nuclear development by North Korea.

"Many actions coming out of North Korea these days are worrisome," Liu said. "We object to any action that is contrary to the goal of the six-party talks. A nuclear North Korea is not beneficial to any nation in this region."

The talks are aimed at getting North Korea to scrap its nuclear program in return for aid and security guarantees. North Korea, which needs donations to help feed its 22 million people, may already have 10 nuclear weapons, the Brussels-based International Crisis Group estimated in November.

The disarmament talks have been at an impasse since the North Korean leadership in Pyongyang admitted the nation had broken the 1994 agreement and continued nuclear weapons development. There have been three rounds of talks in Beijing, yielding no agreement.

The U.S. and its allies suspended fuel-oil shipments to the North after the country acknowledged in October 2002 it had a nuclear program in violation of a 1994 treaty. China is North Korea's only supplier of oil.

North Korea said in February 2003 it had reactivated a nuclear power plant in Yongbyon to produce electricity to compensate for the cutoff of fuel oil shipments.

North Korea reiterated it will not return to the six-nation talks while the U.S. remained "hostile" toward the communist nation, which Bush has been labeled an "outpost of tyranny."

The world's most isolated nation said it planned to continue its nuclear weapons program without regard for U.S. warnings of possible sanctions.

North Korea since 1999 has abided by a self-imposed moratorium on flight testing long-range ballistic missiles capable of reaching the western U.S. It tested a short-range missile March 1.