A number of airlines are cancelling flights or re-routing them away from Iran after a US drone was shot down in the Strait of Hormuz.
Iran confirmed it had shot down the drone, saying it had violated Iranian airspace. The US denies this.
The US Federal Aviation Administration (FAA) has issued an emergency order prohibiting US airlines from flying over the Persian Gulf and Gulf of Oman.
United Airlines has suspended its Newark to Mumbai route.
Iran's Islamic Revolutionary Guard Corps (IRGC) shot down the drone on Thursday. It said the downing of the drone sent a "clear message to America".
However the US military insists the drone was over international waters at the time. It condemned the "unprovoked attack" by the IRGC.
The FAA issued a Notice to Airmen on Thursday prohibiting US-registered aircraft from operating over the Persian Gulf and Gulf of Oman.
United Airlines said: "Given current events in Iran, United has conducted a thorough safety and security review between New York/Newark and India (Mumbai) beginning this evening."
Dubai-based Emirates said it would be avoiding "areas of possible conflict".
A spokesperson for the airline said: "We are carefully monitoring the ongoing developments and are in close contact with the relevant government authorities with regards to our flight operations, and will make further operational changes if the need arises."
British Airways says it will adhere to FAA guidance, avoiding Iranian airspace. It says flights will continue to operate using alternative flight paths.
Lufthansa, which flies to Tehran, says it has stopped flying over parts of Iran. Malaysia Airlines says it is avoiding the airspace over the Strait of Hormuz on its flights to and from London, Jeddah and Medina.
Aviation analyst John Strickland told the BBC: "This is in a part of the world that is a major confluence of traffic. It really is a big artery of air travel, not only to the Gulf itself, but on overflights to places like India and other parts of Asia.
"Given that there is some airspace which is off limits ordinarily because of military usage and that there are these pressure points, it's a challenge and certainly leads to delays and operational headaches for airlines to actually implement that."
Which airlines are changing their routes?
According to Reuters, these major airlines have altered their flight paths:Cathay Pacific Emirates FlyDubai British Airways KLM Qantas Singapore Airlines Lufthansa