The U.S. has warned Iran's Supreme Leader Ayatollah Ali Khamenei through a secret channel of communication that closing the Strait of Hormuz would elicit an American military response, a report says.

United States government officials would not describe how the message was relayed, and whether Iranian leaders responded, the New York Times reports.

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But Obama administration officials have warned Iran that shutting down the Strait of Hormuz — a vital waterway that carries one-sixth of the world's oil supply — would cross a "red line." 

The chairman of the Joint Chiefs of Staff, Gen. Martin E. Dempsey, said recently that if Iran made good on its closure threats, the U.S. would "take action and reopen the strait," which could be accomplished only by military means, including minesweepers, warship escorts and potentially airstrikes, the Times reports.

Japan may reduce oil imports from Iran

The tensions stem from severe new economic sanctions that the U.S., Canada and other Western countries are trying to apply to Iran, in an attempt to halt its suspected nuclear weapons program. Iran, however, says the program is for peaceful purposes. On Wednesday, an Iranian university professor working at a key nuclear facility was killed in a bombing attack.

News of the U.S. warning to Iranian officials via a secret channel comes as Japan's prime minister said Friday the government has yet to decide whether to reduce oil imports from Iran, in line with U.S. sanctions.

Yoshihiko Noda said his government needs to consult with businesses about the sanctions and their implications.

Noda's remarks stood in contrast to that of his Finance Minister Jun Azumi, who said a day earlier that Japan would start reducing Iranian oil imports as soon as possible.

Ahmadinejad meets with Fidel Castro

Meanwhile, Iranian President Mahmoud Ahmadinejad has been on a four-nation Latin America visit, stopping in Cuba to meet with another controversial world figure, Fidel Castro.

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Iranian President Mahmoud Ahmadinejad speaks to a gathering at the University of Havana, in Havana, Cuba, on Wednesday. (Franklin Reyes/Associated Press)

The Iranian president discussed world events with the ailing former Cuban leader for two hours, and declared their two countries to be allies "fighting on the same front."

Cuban President Raul Castro, Fidel's brother, also held a meeting with Ahmadinejad.

"We have common positions on many things," Ahmadinejad said through a translator at an impromptu airport appearance before flying to Ecuador. "We have been, are and will be together, one with the other."

Ahmadinejad began his Latin America trip shortly after Washington imposed tougher sanctions on Tehran over the nuclear program. He spent less than 24 hours in Cuba, following visits to Venezuela and Nicaragua and before heading to Ecuador.

With files from The Associated Press