Iran and the US have both released videos purporting to show an American drone being shot down.
Tehran said the drone was flying over Iranian airspace, which Washington denies.
This incident comes after tensions between the two countries have risen after oil tankers passing the Iranian coast were damaged. The US blames Iran, which denies involvement.
U.S. Central Command posted a tweet with a map of what Guastella said was the drone’s flight path showing it outside of Iran’s territorial waters. "This was an unprovoked attack on a U.S. surveillance asset that had not violated Iranian airspace at any time," he added.
Iranian Foreign Minister Mohammad Javad Zarif said on Twitter the aircraft had taken off from the United Arab Emirates "in stealth mode & violated Iranian airspace."
A US official told AP that Washington had prepared to strike Iran on Thursday night in retaliation for the downing of the drone but that approval was withdrawn before the attacks were launched.
The White House did not answer questions about whether US President Donald Trump had changed his mind.
According to information given to the AP, the strikes were recommendations from the Pentagon and were among options presented to top US officials. It remains unclear how far the preparations had gone.
Earlier in the day, Trump said that “Iran made a very big mistake” by shooting down the drone. But seemed to want to avoid further escalation by saying it could have been carried out by someone who was acting "loose and stupid".
“I find it hard to believe it was intentional if you want to know the truth,” Trump said at the White House. “I think that it could have been somebody who was loose and stupid that did it.”