Iran "made a very big mistake" in shooting down a US military surveillance drone over the Strait of Hormuz, President Donald Trump says.
However, he told reporters it could have been the result of human error, saying: "I find it hard to believe it was intentional."
Iran said the drone had violated Iranian airspace, but the US military denied this.
The incident comes amid escalating tension between the two countries.
Iran's Foreign Minister Javad Zarif said Iran would take its complaint that the US "encroaches on our territory" to the UN.
"We don't seek war but will zealously defend our skies, land and waters," he said on Twitter.
Speaking at the White House, he called the drone's downing a "new fly in the ointment".
Mr Trump said it was "documented" that the unmanned drone had been over international waters and not in Iranian airspace.
"I think probably Iran made a mistake - I would imagine it was a general or somebody that made a mistake in shooting that drone down," he said.
"It could have been somebody who was loose and stupid," he added.
Russia's President Vladimir Putin has warned that war between the US and Iran would be a "catastrophe with unpredictable consequences".
UN Secretary General Antonio Guterres has urged all parties to exercise maximum restraint.
In the US, Democratic House Speaker Nancy Pelosi said the US had no appetite for war with Iran, while the leading candidate for the Democratic presidential nomination, Joe Biden, called Mr Trump's Iran strategy a "self-inflicted disaster".
Meanwhile Saudi Foreign Minister Adel al-Jubeir told the BBC his country was trying to send a message to Iran that its behaviour was "not acceptable".
"Nobody wants to start a war. But we can't let Iran go on a rampage like this. The evidence of Iranian involvement is very compelling. They said they would do it, and now they are doing it," he said.
Oil prices have jumped by about five percent following the incident.
Iran's Islamic Revolutionary Guard Corps (IRGC) said its air force had shot down a US "spy" drone in the early hours after the unmanned aircraft violated Iranian airspace near Kuhmobarak in the southern province of Hormozgan.
In a speech on Iranian state TV, IRGC commander-in-chief Maj-Gen Hossein Salami said the drone's downing was a "clear message" to the US that Iran's borders were "our red line".
The US military's Central Command later confirmed that a US Navy Broad Area Maritime Surveillance (BAMS-D) aircraft was shot down by an Iranian surface-to-air missile system while operating in what it said was international airspace over the Strait of Hormuz at approximately 4.05am Iran time on Thursday (11.35am NZT).
"Iranian reports that the aircraft was over Iran are false," spokesman Navy Capt Bill Urban said. "This was an unprovoked attack on a US surveillance asset in international airspace."
Lt Gen Joseph Guastella, Commander of US Air Forces Central Command, later said the drone had been operating at high altitude and was about 34km from the nearest point on the Iranian coast when it was shot down.
US naval assets have been dispatched to the drone debris field in international waters, a US source told Reuters.
The BAMS-D is a RQ-4A Global Hawk High-Altitude, Long, Endurance (HALE) drone that can carry out surveillance and reconnaissance missions over vast ocean and coastal regions, according to the US military.