At least seven people were killed Sunday night in an explosion at a steel mill in the Iranian city of Yazd. Foreign nationals, possibly North Korean nuclear arms experts, are believed to be among the dead.

The explosion follows two blasts that occurred in Iran in recent weeks at sites linked to Tehran's nuclear program.

The Ghadir steelworks was opened by Iranian President Mahmoud Ahmadinejad some six months ago, and the factory's link to Tehran's nuclear development program remains unclear. According to reports in recent months, however, the Iranians are struggling to produce steel of the grade required for the construction of centrifuges for the enrichment of uranium. Some of the reports note that the new factory includes a closed military zone used for the production of an unknown material.

The cause of Sunday night's explosion remains unknown, with Iranian reports initially putting it down to water getting into the facility. Subsequent reports said the blast was caused by munitions that had accidentally found their way into steel that was being recycled in the plant.

There is no evidence linking the three blasts, and no one has claimed responsibility for the explosions, but if the three incidents are indeed connected, the choice of targets indicates a clear attempt to strike at the various links of the Iranian nuclear program chain - the production of raw materials, uranium-enrichment operations, and the development of launch capabilities for missiles with nuclear warheads.

The explosions in the past few months join a series of assassination attempts on Iranian nuclear scientists over the past two years. Tehran has accused Israel and the United States of being behind these attempts.