SEOUL » North Korea’s nuclear test last month wasn’t just a show of defiance and national pride; it also serves as advertising. The target audience, analysts say, is anyone in the world looking to buy nuclear material.

Though Pyongyang has threatened to launch nuclear strikes on the U.S., the most immediate threat posed by its nuclear technology may be North Korea’s willingness to sell it to nations that Washington sees as sponsors of terrorism. The fear of such sales was highlighted this week, when Japan confirmed that cargo seized last year and believed to be from North Korea contained material that could be used to make nuclear centrifuges, which are crucial to enriching uranium into bomb fuel.

The dangerous message North Korea is sending, according to Graham Allison, a nuclear expert at the Harvard Kennedy School: “Nukes are for sale.”

North Korea launched a long-range rocket in December, which the U.N. called a cover for a banned test of ballistic missile technology. On Feb. 12, it conducted its third underground nuclear test, which got Pyongyang new U.N. sanctions.

Outside nuclear specialists believe North Korea has enough nuclear material for several crude bombs, but they have yet to see proof that Pyongyang can build a warhead small enough to mount on a missile. The North, however, may be able to help other countries develop nuclear expertise right now, as it is believed to have done in the past.

“There’s a growing technical capability and confidence to sell weapons and technology abroad, without fear of reprisal, and that lack of fear comes from (their) growing nuclear capabilities,” Joel Wit, a former U.S. State Department official, said at a recent nuclear conference in Seoul.

Pyongyang says it needs nuclear weapons because of what it calls a hostile U.S. policy aimed at invading the North. The U.S., South Korea and others say North Korean brinksmanship meant to win aid and other concessions is the real motive. Even China, North Korea’s most important ally, opposes its neighbor’s nuclear ambitions.

North Korean nuclear sales earn the impoverished country money that can be pumped back into weapons development, analyst Shin Beomchul at the South Korean-run Korea Institute for Defense Analyses in Seoul said.

Its growing capabilities could make North Korea more attractive to buyers, especially if it is determined that highly enriched uranium was used in last month’s test.

Proliferation worries have ramped up since late 2010, when North Korea unveiled a long-suspected uranium enrichment operation. North Korea’s first two nuclear tests, in 2006 and 2009, were suspected to be fueled by its limited plutonium stockpile. A crude uranium bomb is easier to produce than one made with plutonium, and uranium production is easier to conceal.

Little is known about North Korea’s uranium program, but Washington and others are keenly interested in whether it is producing highly enriched uranium for bombs and whether uranium was used in the third test — two things suspected, but not yet confirmed, by outsiders.

A nuclear test using highly enriched uranium “would announce to the world — including potential buyers — that North Korea is now operating a new, undiscovered production line for weapons-usable material,” Allison, the Harvard nuclear specialist, wrote in a New York Times op-ed after the North’s test.

U.S. officials have hinted that retaliation would follow should Washington discover North Korean cooperation behind any atomic attack on an American city or U.S. ally.

Pyongyang’s nuclear transfers and any use of weapons of mass destruction “would be considered a grave threat to the United States and our allies, and we will hold North Korea fully accountable for the consequences,” President Barack Obama’s national security adviser, Tom Donilon, said last week.

U.S. officials have long tracked North Korean dealings in nuclear and weapons technology. Sanctions have cut down on missile sales, but Iran and Syria, two countries seen by Washington as rogue actors, may continue to be customers.

In November, the head of Iran’s Atomic Energy Organization proposed observing North Korea’s nuclear test, the Japanese news agency Kyodo reported, citing an unidentified Western diplomatic source privy to Pyongyang-Tehran ties.

North Korea is believed to have helped Syria build what senior U.S. intelligence officials called a secret nuclear reactor meant to produce plutonium. In 2007, Israeli jets bombed the structure in a remote Syrian desert.

Japan’s government said Monday that it has determined that a shipment believed to have originated in North Korea violated U.N. sanctions because it contained material that could be used to make nuclear centrifuges.

The shipment of an aluminum alloy was seized from a Singaporean-flagged ship transiting Tokyo last August. The ship was reportedly bound for Myanmar from the Chinese port of Dalian, although Japanese government officials didn’t confirm Myanmar as the destination.

Japan’s chief government spokesman, Yoshihide Suga, said officials searched the ship because they believed it carried North Korean cargo. News reports said the United States tipped off Japan. Suga said officials had determined in subsequent analyses that the rods were made of an alloy that suggests they were intended for use in a nuclear centrifuge.

Suga said the seizure was the first to be conducted under a law Japan passed in 2010 to clamp down on the movement of materials that could be used for nuclear weapons development being brought into, or exported from, North Korea.

The murkiness of the clandestine nuclear trade is a major worry. It’s difficult to know how a buyer would use atomic material or know-how, or where material could end up after being sold.

“The terrorist threat of an improvised nuclear device delivered anonymously and unconventionally by a boat or a truck across our long and unprotected borders is one against which we have no certain deterrent or defensive response,” Robert Gallucci, a former senior U.S. diplomat who negotiated a U.S.-North Korea nuclear deal used to defuse a nuclear crisis in the 1990s, said late last month in Seoul.

“For Americans, this threat is far greater than the unlikely threat that may someday be posed by North Korean nuclear weapons delivered by a ballistic missile,” he said.

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Eric Talmadge in Tokyo and Hyung-jin Kim in Seoul contributed to this report.