By Jill Dougherty and Pam Benson
More than a month after North Korea tested a nuclear device, the United States is unable to pinpoint whether the regime was able to use uranium to fuel the explosion, a capability that would represent a significantly enhanced nuclear program.
The lack of clarity comes as North Korea ratchets up its bellicose rhetoric each day.
New video broadcast on North Korean television showed the nation's leader, Kim Jong Un, addressing his troops along the border on Monday and issuing a blood-chilling threat, "Throw all enemies into the caldron, break their waists and crack their windpipes." It was the same location he and his late father visited in November 2010, just two days before the North shelled an island, killing four South Koreans.
The bellicose comments have been intensifying over the past months, increasing worry about Kim's unpredictability.
"I am very concerned about what they might do. And they are certainly, if they chose ... could initiate a provocative action against the South," Director of National Intelligence James Clapper told the Senate Intelligence Committee on Tuesday.
North Korea has threatened a pre-emptive nuclear attack not only on South Korea, but on the United States too. Military and White House officials have said the United States can defend against any such threat. President Barack Obama told ABC news this week that he does not think North Korea can make good on the threat.
"They probably can't, but we don't like margin of error," Obama told ABC News.
That ambiguity is not lost on Sen. Dianne Feinstein. "There's perhaps nowhere else on Earth where the capacity to wreak enormous damage is matched by the possibility of North Korea using their nuclear weapons," the chairwoman of the Intelligence Committee said.
Particularly worrisome are the advancements the North Koreans have made in their nuclear and missile programs. In December, they successfully launched a long-range ballistic missile for the first time under what the United States and other Western nations say was the guise of putting a satellite into orbit.
The United States is concerned that a long-range ballistic missile could be fitted with a nuclear warhead and could be capable of striking the mainland, although experts say North Korea does not yet have the expertise to do so.
And then last month, the Pyongyang regime conducted its third underground nuclear test.
U.S. officials tell CNN they have not determined what kind of nuclear device the north tested underground from its Pung-gye-Ri nuclear test site.
A U.S. intelligence official said "nothing has changed" since the Office of the Director of National Intelligence put out a statement within hours of the February 12 nuclear test indicating the test had an explosive yield of several kilotons.
The U.S. Air Force uses sniffer planes to take air samples after a test in hopes of scooping up radioactive particles that will provide more specific details about the contents. The official's comment indicates the United States has obtained no further information that would help determine the type of nuclear material used, and at this late date, probably would not.
The Comprehensive Nuclear-Test-Ban Treaty Organization, an international group that monitors nuclear tests, also said it was unable to get a fix on what fissile material was used by the North Koreans and didn't expect it ever would.
"What we are not sure of is if the North Koreans used plutonium, which they used in their earlier tests or highly enriched uranium for this third test," said Daryl Kimball, the executive director of the Arms Control Association. "If they used highly enriched uranium, that would be very worrisome because it would suggest they have a larger supply of this material that would allow them to build a larger number of nuclear weapons."
Kimball said detection is always iffy. If the chamber where the explosion takes place deep underground is well-sealed, few particles may be released into the atmosphere. And if samples are gathered, they need to be "relatively fresh and very good" to determine whether it was a plutonium-based or uranium-based explosion.
The type of nuclear material matters because North Korea gave up its production of weapons-grade plutonium six years ago and only has a limited amount of the fissile material in its stocks, enough for approximately six bombs, according to experts. After years of denial, the regime acknowledged in 2010 it had an enriched uranium facility. That's problematic because such facilities are hard to detect and could produce large supplies of highly enriched uranium.
Kimball doesn't like the trend. "They don't yet have the capability to deliver a nuclear warhead on a long-range missile that could reach the U.S. Despite whatever they say, they may soon - in the next two to three years with further testing - be able to deliver a nuclear warhead on a ballistic missile that could strike their neighbors," Kimball said.
A North Korean expert believes that Kim Jung Un's regime is playing the international community. "North Korea had done three tests: the ballistic missile, the nuclear and a test of all of us, and the question is, what are we going to do?" said Philip Yun, executive director of the Ploughshares Fund, an anti-nuclear weapons foundation.
The United States and the United Nations have recently imposed additional economic sanctions against North Korea, but so far sanctions have not led to the resumption of talks.
The U.S. special envoy to North Korea says unless the regime is serious about talks and meets its obligations to de-nuclearize, its future will be bleak.
"North Korea will not achieve security, economic prosperity and integration into the international community while it pursues nuclear weapons, while it threatens its neighbors, while it tramples on international norms, abuses its own people and refuses to fulfill its long-standing obligations and commitments," Glyn Davies told a congressional panel. "This is one of the hardest foreign policy problems out there."