Earlier on Thursday, Iran's Revolutionary Guard shot down a US drone over the collapsing nuclear deal with world powers.
White House press secretary Sarah Sanders says Trump was briefed Wednesday night and again Thursday morning about the incident. She says the administration also will keep in touch with lawmakers on Capitol Hill.
Trump made his comment on Twitter in midmorning.
Both Iranian and American officials confirmed the shooting of the drone, but they disputed the circumstances of the incident.
The US military's Central Command released a statement calling the shooting an "unprovoked attack."
Iran’s Revolutionary Guard claimed that the drone shot down was a US "spy drone" which violated Iranian airspace near the Strait of Hormuz.
The downing of the US RQ-4 Global Hawk by Iran comes after the US military previously alleged Iran fired a missile at another drone last week that responded to the attack on the two oil tankers near the Gulf of Oman.
The US had blamed Iran for the attack on the tankers, a claim that Tehran had denied.
Shooting down the drone on Thursday was "a clear message" that Iran will defend its borders, the head of the Revolutionary Guard said.
The tensions between US and Iran are on rise ever since Donald Trump decided to withdraw from Iran’s nuclear deal with world powers a year ago.
Iran has recently increased its production of low-enriched uranium and threatened to boost its enrichment closer to weapons-grade levels.