Ethiopia, June 21 -- 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 U...