Several international airlines were diverting planes from flying over the Strait of Hormuz and parts of Iran on Friday, a day after the Iranian military shot down an American surveillance drone and the United States went to the brink of launching a retaliatory strike.

The Federal Aviation Administration issued an emergency order early Friday that prohibited all American flights in Tehran-controlled airspace above the Persian Gulf and Gulf of Oman because of “heightened military activities and increased political tensions.”

The agency said that flight operations in the area were prohibited “until further notice.”

United Airlines said in a statement that after a security assessment, it had suspended flights between Newark Liberty International Airport in New Jersey and Mumbai, India, that typically fly through Iranian airspace.

The German airline Lufthansa said in an emailed statement that its planes would not fly over the Strait of Hormuz and that the diversion area was likely to expand.