US has warned Iran's Supreme leader Ayatollah Ali Khamenei that any move to close the strategic Strait of Hormuz would amount to crossing the 'red line' that would provoke an American response. The warning was conveyed to the top Iranian brass through a secret channel, The New York Times reported quoting senior US officials, who declined to describe the unusual contact. The NYT said senior Obama administration officials have publicly said that Iran would cross a red line if it made good on recent threats to close the Strait through which 16 million barrels of oil - about a fifth of world's daily oil trade - passes every day. Iran has threatened that it would close the Strait of Hormuz, which is just 45 kms wide at its narrowest point and works as a crucial waterway connecting the Persian Gulf to the Gulf of Oman, in the event of a military strike or severe tightening of international sanctions on its oil exports. The secret communication channel was chosen to underscore privately to Iran the depth of American concern about rising tensions over the Strait, where US Naval officials say their biggest fear is that an overzealous Revolutionary Guards' captain could indulge in provocative act, triggering a crisis.