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.
Iran's UN ambassador Majid Takht Ravanchi said the drone was engaged in a clear spying operation in what he called a blatant violation of international law.
In a letter to the UN secretary-general and the Security Council, Ravanchi said that while Iran was not seeking war, it reserved the right to defend its territory against hostile acts.
Speaking at the White House, he called the drone's downing a "new fly in the ointment".
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 Trump's Iran strategy a "self-inflicted disaster".
The top Democrat in the US Senate, Chuck Schumer, said: "The president may not intend to go to war here, but we're worried that he and the administration may bumble into a war."
The Republicans' leader in the House, Kevin McCarthy, called for a "measured response" to the incident.
Meanwhile Saudi Foreign Minister Adel al-Jubeir said 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 5% following the incident.
United Airlines has suspended its Newark to Mumbai flights, which pass through Iranian airspace, after "a thorough safety and security review".
BBC
BDST: 1030 HRS, JUNE 21, 2019
RS
