On matters nuclear and some priorities for the US

WASHINGTON, April 28 - The head of the Defense Intelligence Agency said Thursday that American intelligence agencies believed North Korea had mastered the technology for arming its missiles with nuclear warheads, an assessment that if correct, means the North could build weapons to threaten Japan and perhaps the western United States.

While Vice Adm. Lowell Jacoby, the Defense Intelligence Agency chief, said in Senate testimony that North Korea had been judged to have the "capability" to put a nuclear weapon atop its missiles, he stopped well short of saying it had done so, or even that it had assembled warheads small enough for the purpose. Nor did he give evidence to back up his view during the public session of the Senate Armed Services Committee.

Still, his assessment of North Korea's progress exceeded what officials have publicly declared before.

When asked by Senator Hillary Rodham Clinton of New York during a hearing on Thursday whether "North Korea has the ability to arm a missile with a nuclear device," Admiral Jacoby responded, "The assessment is that they have the capability to do that, yes ma'am."

At a White House news conference on Thursday, President Bush said that given the uncertainties, he was worried about the progress North Korea had made on its nuclear program under its leader, Kim Jong Il. "There is concern about his capacity to deliver," he said. "We don't know if he can or not, but I think it's best when dealing with a tyrant like Kim Jong Il to assume that he can."

In 2003, the United States warned South Korea and Japan that satellite imagery had identified an advanced nuclear testing site in a remote corner of North Korea where equipment had been set up to test conventional explosives that could compress a plutonium core and set off a compact nuclear explosion.

Since then, American investigators have been pressing Pakistan for details about the kind of technology North Korea might have been given, perhaps in conjunction with visits to Pakistani nuclear sites. North Korea supplied Pakistan with many missiles it for its nuclear arsenal.

Building a nuclear warhead that can be delivered by a missile requires the technical sophistication to make it small and light. North Korea has never conducted a test that would prove it could manufacture a warhead, though in recent days anxiety has risen in Washington and among North Korea's Asian neighbors that the country could conduct a test in an effort to force the world to deal with it as a nuclear state.

To field a working nuclear missile, North Korea would also have to conduct new tests of its missiles themselves and of their payloads, including such complex components as heat shields for re-entry of the warhead. North Korea's last significant missile test, in 1998, overshot Japan and would not have been able to reach United States territory.

North Korea is considered one of the most opaque intelligence targets for American analysts, and the absence of reliable human spies has made it more difficult to understand the progress of its program.

Admiral Jacoby said North Korea's ability to deliver a nuclear warhead to the continental United States remained "a theoretical capability" because its Taepo Dong 2 missile had not been flight tested. But he added that American intelligence agencies judged that a two-stage Taepo Dong could strike parts of the American West Coast and that a three-stage variant could probably reach all of North America.

In an interview on Thursday, Mrs. Clinton called Admiral Jacoby's statement "the first confirmation, publicly, by the administration that the North Koreans have the ability to arm a missile with a nuclear device that can reach the United States," adding, "Put simply, they couldn't do that when George Bush became president, and now they can."

At his news conference, Mr. Bush defended his decision to pursue the talks in an effort to stop North Korea's nuclear program and noted that the United States was exploring options including taking the issue to the United Nations Security Council, if the North did not return to the talks.

"It's better to have more than one voice sending the same message to Kim Jong Il. It's the best way to deal with this issue diplomatically," he said. "We'll continue to do so."

In a statement, a Defense Intelligence Agency spokesman, Donald Black, said Admiral Jacoby "was reiterating" testimony he gave last month before the committee, in which he said the Taepo Dong 2 intercontinental ballistic missile "may be ready for testing," adding, "This missile could deliver a nuclear warhead to parts of the United States." He did not say then that the North Koreans were able to make a warhead that the missile could hurl such a distance.

Analysts with experience in Asia said the importance of Admiral Jacoby's conclusion was striking.

"This has to constrain the president's ability to deal with the North Korean nuclear problem," said Jonathan Pollack, a professor of Asian and Pacific Studies at the Naval War College who has written extensively on the North's program. "If you believe that Japanese territory is potentially at risk to a North Korean nuclear-armed missile, it has to change the calculation."

If Mr. Bush accepts that judgment, it could significantly complicate choices he must make in the next several months. North Korea declared publicly for the first time in February that it had nuclear weapons. This month, American spy satellites detected that the North had shut down its nuclear power plant at Yongbyon and could be preparing to reprocess its spent fuel, a move that could result in the production of enough plutonium to build up to three more nuclear bombs.

Admiral Jacoby said American intelligence agencies had increased their assessment of the current North Korean arsenal's size, but he gave no numbers. Other government officials, in interviews off the record, have estimated that North Korea's arsenal has increased by six weapons' worth of plutonium since the North threw international inspectors out of the country in early 2003, and began turning a stockpile of 8,000 spent fuel rods into plutonium.

The six-nation talks have been stalled since last June. China has played host to three inconclusive rounds of the negotiations, which involved the United States, North and South Korea, China, Japan and Russia.