President Donald Trump said Thursday that Tehran had made a "very big" error, after Iranian forces shot down a US spy drone near the strategic Strait of Hormuz.
"Iran made a very big mistake!" he tweeted in his first public reaction to the strike.
Iran's Islamic Revolutionary Guard Corps earlier said it had brought down the surveillance drone after it entered its country's airspace. The Pentagon said the incident occurred in international airspace.
The drone shooting adds to growing tensions as Tehran pushes back against surging US diplomatic, economic and military pressure.
Trump has repeatedly said he does not favour war with Iran unless it is to stop the country getting a nuclear weapon -- something Iranian leaders insist they are not pursuing.
Critics of the Trump administration say that his policy of "maximum pressure" -- including crippling economic sanctions, abandonment of a complex international deal to regulate Iran's nuclear activities, and deployment of extra sea, air and land forces to the region -- make war ever more likely.
The drone downing came as Iran was already accused by Washington of having carried out explosions on two oil tankers in the congested Hormuz area. Tehran denies having been behind the attacks but has frequently threatened in the past to block the sea lanes used by shipping to move much of the world's oil exports.