(CNN) -- The U.S. has used multiple means of reaching out to Iran in response to Iran's threat to blockade the Strait of Hormuz, a critical transit point for one-fifth of the world's oil.
Among those messages, Washington sent a rare letter to the Iranian government, delivered by U.S. Ambassador to the United Nations Susan Rice to Ambassador Mohammad Khazaee, Iran's permanent representative at the U.N., according to a U.S. official who would only speak on the condition of anonymity. The official would provide no further details
Iranian media reported that Iran is studying the letter and "will respond if necessary," quoting Foreign Ministry Spokesman Ramin Mehmanparast.
In addition, as reported previously, Defense Secretary Leon Panetta publicaly warned Tehran against such a blockage move last Thursday, threatening to "respond" if Iran attempts to shut down traffic. The United States also sent diplomatic messages through Switzerland -- its protecting power in the country -- and Iraqi President Jalal Talibani.
The messages came as tensions in the region continued to escalate. On Friday, the Navy and Coast Guard released video of two incidents on January 6, when Iranian boats came extremely close to U.S. vessels.