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WASHINGTON - The Obama administration is relying on a secret channel of communication to warn Iran's supreme leader, Ayatollah Ali Khamenei, that closing the Strait of Hormuz is a "red line" that would provoke an American response, according to U.S. government officials.

The officials declined to describe the unusual contact between the two governments, and whether there had been an Iranian reply. Senior Obama administration officials have said publicly that Iran would cross a line if it made good on recent threats to close the strait, a strategically crucial waterway connecting the Persian Gulf to the Gulf of Oman, where 16 million barrels of oil -- about a fifth of the world's daily oil trade -- flow through every day.

Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, said last weekend that the United States would "take action and reopen the strait," which could be accomplished only by military means, including minesweepers, warship escorts and, potentially, airstrikes. Defense Secretary Leon Panetta told troops in Texas on Thursday that the United States would not tolerate Iran's closure of the strait.

The secret communications channel was chosen to underscore privately to Iran the depth of U.S. concern about rising tensions over the strait, where U.S. naval officials say their biggest fear is that an overzealous Revolutionary Guards naval captain could do something provocative on his own, triggering a larger crisis.

"If you ask me what keeps me awake at night, it's the Strait of Hormuz and the business going on in the Arabian Gulf," Adm. Jonathan Greenert, the chief of naval operations, said in Washington this week.

Administration officials and analysts said they believe that Iran's threats to close the strait, coming amid deep frictions over Iran's nuclear program and possible sanctions, were bluster and an attempt to drive up the price of oil. Blocking the route for the vast majority of Iran's petroleum exports -- and for its food and consumer imports -- would amount to economic suicide.

"They would basically be taking a vow of poverty with themselves," said Dennis Ross, who until last month was one of President Obama's most influential advisers on Iran. "I don't think they're in such a mood of self-sacrifice."

But Pentagon officials, who plan for every contingency, said that, however unlikely, Iran does have the military capability to close the strait.

'The simple answer is yes'

Although Iran's naval forces are hardly a match for those of the United States, for two decades Iran has been investing in the weaponry of "asymmetric warfare" -- mines, fleets of heavily armed speedboats and anti-ship cruise missiles hidden along Iran's 1,000 miles of Persian Gulf coastline -- which have become a threat to the world's most powerful navy.

"The simple answer is yes, they can block it," Dempsey said on CBS on Sunday.

Estimates of how long it could take for U.S. forces to reopen the strait range from a day to several months, but the consensus is that while Iran's naval forces could inflict damage, they would ultimately be destroyed.

"Their surface fleet would be at the bottom of the ocean, but they could score a lucky hit," said Michael Connell, the director of the Iranian studies program at the Center for Naval Analysis, a research organization for the Navy and Marine Corps. "An anti-ship cruise missile could disable a carrier."

Iran has two navies: one, its traditional state navy of aging big ships dating back to the era of the shah, and the other a politically favored Revolutionary Guards navy of fast-attack speedboats and guerrilla tactics. Senior U.S. naval officers say the Iranian state navy is for the most part professional and predictable, but the Revolutionary Guards navy, which has responsibility for the operations in the Persian Gulf, is not.

'Probability for buffoonery'

"You get cowboys who do their own thing," Connell said. One officer with experience at the Navy's Fifth Fleet headquarters in Bahrain said the Revolutionary Guards navy shows "a high probability for buffoonery."

That navy has been steadily building and buying faster missile boats and stockpiling what U.S. experts say are at least 2,000 naval mines. Many are relatively primitive, about the size of a garbage can, and easy to slip into the water. "Iran's credible mining threat can be an effective deterrent to potential enemy forces," an unclassified report by the Office of Naval Intelligence, the U.S. Navy's intelligence arm, said in 2009.

Although the United States would respond with minesweepers, analysts said U.S. naval forces might encounter layers of simultaneous attacks. The Iranians could launch anti-ship missiles from their coastline, islands or oil platforms and at the same time surround any U.S. ship with missile-armed speedboats.

The tight squeeze of the strait, which is less than 35 miles wide at its most narrow point, offers little maneuvering room for warships. "It would be like a knife fight in a phone booth," said a senior Navy officer.

The strait's shipping lanes are even narrower: Both the inbound and outbound lanes are 2 miles wide, with a 2-mile-wide stretch separating them.

In 2002, a classified, $250 million Defense Department war game concluded that small, agile speedboats swarming a naval convoy could inflict devastating damage on more powerful warships. Lt. Gen. Paul Van Riper, a retired Marine Corps officer who served in the war game, said in 2008: "The whole thing was over in 5, maybe 10 minutes."