A blast caused by leftover ammunition killed eight workers at a steel mill in the central Iranian city of Yazd late on Sunday, the official Islamic Republic News Agency reported on Monday. The agency said the blast at the privately owned factory wounded 12 other workers. Although the report said several foreign nationals had been killed, it did not state their nationality or how many foreigners were injured.

Analysts have surmised that these "foreign nationals" could be from North Korea, which provides Iran (and Syria) with a unique type of steel, as reported by the German newspaper Die Welt two weeks ago. The newspaper said the steel could be used by the Iranians for manufacturing missile parts as well as centrifuges for enriching uranium.

This type of steel, used to build key components of gas centrifuges, is on the monitoring list of the Nuclear Suppliers Group and the Missile Technology Control Regime, and its export is prohibited to Iran under international sanctions. Iran purchased large quantities of the steel before the sanctions were imposed, and continues to do so. According to the American non-profit organization Institute for Science and International Security, Iran has secretly purchased 67 tons of this steel from Britain -- enough to build 33 centrifuges.

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Local parliamentarian Ali Akbar Olya told the Iranian Labor News Agency that the explosion in the central province of Yazd happened when scrap metal was being melted down. "Among those steel scraps were obsolete munitions which caused the explosion," he said.

The scrap metal had been gathered in western Iran, Olya said, the scene of the 1980-88 Iran-Iraq war where old military scrap from the conflict still remains.

Yazd Governor Azizulah Seifi told the semi-official Fars news agency that "the plant belongs to the private sector and its directors and officials have been summoned."

Sunday's blast comes on the heels of a string of mysterious explosions in Iran in recent weeks. In late November, satellite images indicated that an explosion had occurred at a nuclear facility near the city of Isfahan, despite vehement denials by Iranian officials. The unexplained blast came two weeks after an explosion at a Revolutionary Guard weapons depot near the Iranian capital killed 17 soldiers, including a senior Revolutionary Guard commander. The cause of these explosions remains unclear.

Initially Sunday's blast was reported to have been caused not by defunct ammunition but by water seepage. Conflicting reports such as this, reminiscent of the confusion that followed the Isfahan explosion, contribute to suspicion that Iran is hiding something.

Explosions at Iranian industrial sites are occasionally reported by the media, and are mostly blamed on old equipment, lack of maintenance and inappropriate technology. Some officials have blamed the poor maintenance on economic sanctions imposed on Iran by the U.S. and other countries over Tehran's disputed nuclear activities. Iran denies Western allegations that it is seeking to build nuclear weapons.

Meanwhile, Iran issued a harsh threat on Monday, with Iranian parliament member Parviz Sarvari saying the military was set to practice closing the Persian Gulf to shipping at the narrow Strait of Hormuz, the most important oil transit channel in the world.

"Soon we will hold a military maneuver on how to close the Strait of Hormuz. If the world wants to make the region insecure, we will make the world insecure," Sarvari said. There was no official confirmation of his remarks.

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