Iran’s official news agency, IRNA, confirmed Sunday that Iran has received a letter from the United States Ambassador to the United Nations.

According to AFP,  the Iranian agency quoted Iranian Foreign Ministry Spokesman Ramin Mehmanparast as saying, “The U.S. ambassador to the U.N., Susan Rice, sent a letter to Mohammad Khazaie, Iran’s U.N. representative, which was conveyed by the Swiss ambassador, and finally Iraqi President Jalal Talabani delivered its contents to officials” in Iran.”

“We are in the process of studying the letter and if necessary we will respond,” he added.

Since 1979, the United States has no diplomatic relations with Iran, and the U.S. has often used Switzerland as a neutral party to relay diplomatic messages to Tehran. This time, however, a different channel was used.

On Friday, the New York Times quoted unnamed U.S. officials in reporting that Washington had used a secret channel to warn Iran’s supreme leader Ayatollah Ali Khamenei that closing the Hormuz Straits would cross a “red line” and provoke a response.

“The secret communications channel was chosen to underscore privately to Iran the depth of American concern about rising tensions over the strait,” the newspaper said.

Pentagon officials said that while Iran’s naval forces are hardly a match for those of the United States, it has the military capability to close the strait with mines, fleets of heavily armed speed boats and anti-ship cruise missiles hidden along the Persian Gulf coastline. The U.S. Joint Chiefs of Staff chairman General Martin Dempsey has also said Iran could close the Straits, but that the U.S. would then reopen them.

The Times added that American officials had indicated that “the recent and delicate messages expressing concern about the Strait of Hormuz were conveyed through a channel other than the Swiss government.”