WASHINGTON — US officials and independent analysts say North Korea appears to have taken unusual steps to conceal details about the nuclear weapon it tested last month, fueling suspicions that its scientists shifted to a bomb design that uses highly enriched uranium as the core.
At least two separate analyses of the Feb. 12 detonation confirmed that the effects of the blast were remarkably well contained, with few radioactive traces escaping into the atmosphere where they could be detected, according to US officials and weapons experts who have studied the data.
US officials anticipated the test and monitored it closely for clues about the composition of the bomb, which was the third detonated by North Korea since 2006. The first two devices were thought to have used plutonium extracted from a dwindling stockpile of the fissile material that North Korea developed in the late 1990s.
A successful test of a uranium-based bomb would confirm that Pyongyang has achieved a second pathway to nuclear weapons, using its plentiful supply of natural uranium and new enrichment technology. A device based on highly enriched uranium — HEU — also would deepen concerns about cooperation between the hermetic regime and Iran.
North Korea’s belligerent threats in recent weeks have increased concerns among American and South Korean officials and ratcheted up worries about the level of progress on long-range missiles and nuclear weapons by Pyongyang.
On Sunday, the North Korean leader, Kim Jong Un said his country was determined to rebuild its economy despite international sanctions and also expand its nuclear weapons arsenal, which the ruling party called ‘‘the nation’s life.’’
There are two paths to a nuclear weapon. The bomb that the United States dropped on Hiroshima in 1945 used HEU as its core, and the one dropped three days later on Nagasaki was a plutonium device. North Korea has long possessed plutonium, but its enrichment of uranium is a more recent development.
Iran has been concentrating on uranium enrichment, which it says is for civilian purposes.
Although North Korea and Iran have cooperated on missile technology, US officials said there is no direct evidence of nuclear cooperation.
‘‘We’re worried about it, but we haven’t seen it,’’ said a former Obama administration official, who spoke on the condition of anonymity to discuss intelligence assessments. ‘‘They cooperate in many areas, especially missiles. Why it hasn’t yet extended to the nuclear program is frankly a mystery.’’
The prospect of a third nuclear test prompted heightened scrutiny of the Korean Peninsula by intelligence agencies from the United States and other countries.