United Airlines has suspended its flights from Newark to Mumbai. (File)

The US Federal Aviation Administration on Thursday issued an emergency order prohibiting US operators from flying in an overwater area of Tehran-controlled airspace over the Strait of Hormuz and Gulf of Oman due to heightened tensions.

The order came hours after United Airlines suspended flights between New Jersey's Newark airport and Mumbai, which fly through Iranian airspace, following a safety review after Iran shot down a high-altitude US surveillance drone.

United said it had suspended its flights to India through Iran airspace after a "thorough safety and security review". It did not say how long the suspension would last.

A United spokesman said customers flying from Mumbai to Newark would be booked on alternative flights back to the United States.

"We continue to explore all our options and remain in close contact with relevant government authorities in order to provide our customers with the most efficient travel experience under these circumstances," the spokesman said.

On Thursday, two other carriers, American Airlines and Delta Air Lines, said they did not fly over Iran. Japanese carriers Japan Airlines Co Ltd and ANA Holdings Inc also said they did not fly over the area.

The downing of the unarmed Global Hawk aircraft, which can fly at up to 60,000 ft (18,300 m), was the latest of a series of incidents in the Gulf region, a critical artery for global oil supplies, that included explosive strikes on six oil tankers.

FAA said according to flight tracking applications, the nearest civil aircraft was operating within around 45 nautical miles of a US Global Hawk drone when it was shot down by an Iranian surface-to-air missile.

"There were numerous civil aviation aircraft operating in the area at the time of the intercept," FAA said.

The agency said it remained concerned about the escalation of tension and military activity within close proximity to high volume civil aircraft routes as well as Iran's willingness to use long-range missiles in international airspace with little or no warning.

In July 2014, Malaysia Airlines Flight MH17 was shot down by a missile over Ukraine, killing all 298 on board, prompting carriers to take more steps to uncover threats to their planes.

The US prohibition does not apply to airlines from other countries, but OPSGROUP, which provides guidance to operators, said it would be taken into consideration by carriers globally.

"Since MH17, all countries rely on advice from the US, the UK, France and Germany to highlight airspace risk," OPSGROUP said. "The threat of a civil aircraft shootdown in southern Iran is real."

Flight tracking website Flightradar24 showed Qatar Airways and Etihad Airways flights in the area prohibited for US carriers at 0300 GMT on Friday. Qatar and Etihad did not respond immediately to a request for comment outside usual business hours.