The New York Times is quoting unnamed U.S. officials as saying Washington has sent a message to Iran’s Supreme Leader, Ayatollah Khamenei, that closing the Strait of Hormuz would provoke a U.S. response.

The report published late Thursday said the covert message was not delivered through the the Swiss government, the usual way Washington communicates with Iranian leaders.

Iran recently threatened to close the narrow waterway, a vital shipping route for the global oil trade. U.S. Secretary of State Hillary Clinton said earlier this week that the threat is “provocative and dangerous.”