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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. Mr. 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.

Mr. 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.

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