WASHINGTON (AP) — North Korea has followed through on its threat to advance its nuclear weapons program, the top U.S. intelligence official said Wednesday, while a research institute pointed to signs the communist country is preparing to launch bigger rockets.

Those developments will add to the international concern about the intentions of young leader Kim Jong Un, amid scant sign that negotiations to curb North Korea's nuclear ambitions will resume any time soon.

In written testimony to the Senate Intelligence Committee, Director of National Intelligence James Clapper said North Korea has expanded the size of the uranium enrichment facility at the Nyongbyon nuclear complex and restarted a reactor that was used for plutonium production before it was shut down in 2007.

Those findings concur with assessments published last summer by think tanks that monitor North Korea's nuclear program using commercial satellite imagery. South Korean intelligence also has said the reactor has restarted.

"North Korea's nuclear weapons and missile programs pose a serious threat to the United States and to the security environment in East Asia," Clapper said. He said the North is committed to developing a long-range missile that can threaten the U.S.

North Korea announced its intention to "adjust and alter" its existing nuclear facilities after an underground, atomic test explosion last February, backtracking from denuclearization commitments. That announcement came during a period of high tension when the North issued threats of a nuclear strike on the United States.

Tensions have eased some since then, and the North says it wants to improve ties with South Korea. The North has said it is willing to resume, without preconditions, the six-nation aid-for-disarmament talks from which it withdrew in 2009.

But North Korea is denouncing upcoming U.S.-South Korean military exercises, and appears to be moving away from the U.S. demand for a concrete demonstration of its commitment to denuclearization before the talks can restart.

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Satellite photos from this month show the North is modifying its main launch site to handle rockets 10 meters taller than the 30-meter one it fired into space a year ago, according to an analysis by 38 North, the website of the U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies.

Because of the construction, the launch pad will not be available for launches until March or April at the earliest, the analysis says.

The launch of a three-stage Unha-3 rocket into space from the west coast site of Sohae in December 2012 was viewed by the U.S. as a worrying sign in the North's development of ballistic missile technology, with the North moving closer toward the goal of a nuclear-tipped, missile that can reach America. North Korea said the launch was part of a peaceful space program.

In its analysis, 38 North detected construction of various facilities at Sohae that suggest North Korea is preparing it to handle the road-mobile intercontinental ballistic missile, known as the KN-08. There may have been a test of a KN-08 rocket engine in the past month, the analysis said.

"North Korea's activities give us advanced warning of a significant escalation in Pyongyang's production of weapons of mass destruction," said Joel Wit, a former State Department official and editor of 38 North.

Clapper said Wednesday that the North has taken initial steps toward fielding the KN-08 although it remains untested. Some independent analysts have said the KN-08s displayed at parades in Pyongyang, the capital, could have just been mock-ups.

Assessing the intentions of North Korea's secretive government and the nation's technical capabilities is notoriously difficult. 38 North's conclusions are based on imagery analysis by Nick Hansen, a retired intelligence expert who closely monitors developments in the North's weapons programs.