North Korea said on Thursday it would restart its weapons-grade nuclear program if the United Nations takes any action to punish it for firing a rocket that Pyongyang claims is a satellite, Reuters reported.

North Korea mounted a rocket on a launchpad on its northeast coast, American officials said, putting the country well on track for a launch the U.S. and South Korea warned Thursday would be a major provocation with serious consequences.

Pyongyang says the rocket will carry a satellite, but regional powers suspect the North will use the launch to test the delivery technology for a long-range missile capable of striking Alaska. They have said the launch — banned by the U.N. Security Council in 2006 — would trigger sanctions.

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"The moment the Sept. 19 joint statement is ignored due to such act the six-party talks will come to an end, all the processes for the denuclearization of the Korean Peninsula ... will be brought back to what used to be before their start and necessary strong measures will be taken," Reuters quotes North Korea's foreign ministry spokesman on the plutonium program.

The U.S. Navy is moving two warships with technology to track and destroy missiles into the area, and South Korea said it is deploying a destroyer to the waters off Japan, AFP reported.

"I would say we are ready for any contingencies," AFP quotes U.S. Navy public relations officer Charles Howard.

The USS McCain and USS Chafee left Sasebo port in southwestern Japan Thursday.

The Defense Department said it was monitoring the situation closely and would oppose the prepared launch.

"Whether it is a satellite launch or a missile launch, either way ... it would be in violation of the U.N. Security Council resolution 1718, and therefore we would, of course, oppose it," Pentagon spokesman Geoff Morrell said Wednesday.

U.S. Secretary of State Hillary Rodham Clinton warned such a "provocative act" could jeopardize the stalled talks on supplying North Korea with aid and other concessions in exchange for dismantling its nuclear program.

"We have made it very clear that the North Koreans pursue this pathway at a cost and with consequences to the six-party talks, which we would like to see revived," Clinton said Wednesday in Mexico City.

"We intend to raise this violation of the Security Council resolution, if it goes forward, in the U.N.," she said. "This provocative action in violation of the U.N. mandate will not go unnoticed, and there will be consequences."

North Korea declared last month that it was making "brisk headway" in preparations to send its Kwangmyongsong-2 communications satellite into space, and notified aviation and maritime authorities of a time frame for the launch: April 4-8, between 11 a.m. and 4 p.m.

Commercial satellite imagery from DigitalGlobe revealed steady progress toward a launch, with a flurry of activity at the Musudan-ni site in late February and an open hatch and crane hovering above the launchpad two weeks ago, Jane's Intelligence Review editor Christian Le Miere said. After mounting the rocket, scientists would need a number of days to conduct tests and to fuel the projectile.

U.S. spy satellites spotted the rocket two days ago, South Korean reports said — the first indication that the countdown toward a launch has begun. Counterterrorism and intelligence officials in Washington confirmed reports that a rocket was in position.

North Korea is now "technically" capable of launching it in three to four days, South Korea's Chosun Ilbo newspaper said, citing an unnamed diplomatic official.

However, South Korean and U.S. intelligence authorities have not yet determined whether the rocket is intended to carry a satellite or a missile because the top is concealed, the Yonhap news agency said, citing an unnamed South Korean government official.

North Korea calls the rocket the Unha-2, which experts say utilizes the same delivery system as the Taepodong-2 long-range ballistic missile. The Chosun Ilbo described the rocket as the Taepodong-2 based on its size.

The government said Thursday it could not confirm reports the rocket was in place. But Seoul urged the North to cancel the launch, warning that the move would threaten regional stability and draw international sanctions.

"If North Korea pushes ahead with the launch by ignoring repeated warning by our government and the international community, that would be a serious challenge and provocation on security on the Korean peninsula and regional stability in Northeast Asia," ministry spokesman Won Tae-jae told reporters.

South Korea will take North Korea to the U.N. Security Council whether it launches a satellite or a missile, Foreign Ministry spokesman Moon Tae-young said.

The Associated Press, AFP and Reuters contributed to this report.