Here's an excerpt from a New York Times article about it.
TEHRAN — Iran warned Monday that it could easily close a critical Persian Gulf waterway for oil shipments and claimed possession of a new long-range naval weapon that could sink enemy ships nearly 200 miles away.
It was unclear what provoked the warning, made by Iran’s Revolutionary Guards, but it followed the weekend expiration of an informal deadline for Iran to respond to incentives from world powers to curb its uranium enrichment activities. The United States, which has warships deployed in the Persian Gulf, has said new sanctions should be imposed on Iran for failing to respond to the deadline.
The Iranian warning coincided with word that Iran’s chief nuclear negotiator, Saeed Jalili, spoke by phone on Monday with the European Union’s foreign policy chief, Javier Solana. Mr. Solana was expected to report back on the conversation to the representatives of the six countries — the United States, Russia, China, France, Britain and Germany — that are leading the demands that Iran stop enriching uranium, Agence France-Presse reported.
In comments carried by the semiofficial Iranian news agency Fars, Gen. Mohammad Ali Jafari, the head of Iran’s Revolutionary Guards, said that Iran was capable of imposing “unlimited controls” at the Strait of Hormuz in the Persian Gulf, an important international oil route.
“Closing the Strait of Hormuz for an unlimited period of time would be very easy,” he was quoted as saying.