The United States insisted that it would not tolerate a nuclear-armed North Korea, as reports suggested that the isolated regime was planning to launch another long-range rocket capable of striking the US.

South Korea’s Yonhap news agency quoted unnamed intelligence sources saying that a missile-like object has been spotted on satellite photographs being removed from a missile factory near the North Korean capital, Pyongyang.

Similar reports in February were followed two months later by the launch of a long-range rocket which was fired into the Pacific, drawing condemnation from governments around the world and the censure of the UN Security Council.

An even stronger protest from the UN is expected this week in response to North Korea’s underground test of a nuclear weapon last Monday, followed by a spate of short range missile tests.

On Saturday, the US defence secretary, Robert Gates, told an audience in Singapore that the US will not accept North Korea as a nuclear state, although he gave no concrete details of how the Obama administration intended to prevent this.

“I think that the combination of their progress in developing nuclear technology, and their progress in developing multistage long-range missiles, is a harbinger of a dark future,” he said.

“What is now central to multilateral efforts ... is to try to peacefully stop those programs before they do in fact become a ’clear and present danger’.”

He said that the transfer of North Korean nuclear technology to foreign countries or terrorist groups would be regarded as a “grave threat”, and that there would be no reward for threats.

“We will not accept North Korea as a nuclear weapons state,” he said. “Everyone ... is familiar with the tactics that the North Koreans use. They create a crisis and the rest of us pay a price to return to the status quo ante. As the expression goes in the United States, ’I am tired of buying the same horse twice.’”

But if Pyongyang is already rolling out a new long range rocket, it confirms the growing conviction among North Korea-watchers in Seoul that, rather than tactical provocations intended to compel the attention of a new US government, its recent actions are part of an accelerated drive to perfect a strategic nuclear deterrent.

“In the past, these kind of actions have been separated by a few months, and so the US has time to react and respond,” says Choi Jin Wook, of the Korea Institute for National Unification. “But this time they are playing all their cards at once. It suggests that something new is taking place."

North Korea insisted that the long range rocket which it fired in April was the vehicle for a communications satellite. But the US, South Korea and Japan say that it also has the potential to carry a missile warhead across the Pacific, potentially putting Alaska and Hawaii within range.

The Yonhap source said that the process of preparing and fuelling the rocket could be completed in as little as two weeks, with the projectile ready to go up in mid-June at around the time of a planned summit meeting between President Obama and the South Korean president, Lee Myung Bak.

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