A US military drone has been shot down by an Iranian surface-to-air missile while in international airspace over the Strait of Hormuz, US officials say.
One official told Reuters news agency the drone was a US Navy MQ-4C Triton.
Earlier, Iran's Islamic Revolution Guards Corps (IRGC) said it had shot down a US drone over Iranian airspace, near Kuhmobarak in the southern province of Hormozgan.
It comes at a time of escalating tension between the US and Iran.
On Monday, the US defence department said it was deploying 1,000 extra troops to the region in response to "hostile behaviour" by Iranian forces.
The US has accused Iran of attacking oil tankers with mines in the Gulf of Oman, which is just outside the Strait of Hormuz. Iran says the US claims are fabricated.
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The Iran nuclear crisis in 300 words
Tensions were further fuelled on Monday when Iran said its stockpile of low-enriched uranium would next week exceed limits it agreed with world powers under a landmark nuclear deal in 2015.
Iran stepped up its production in response to tightening economic sanctions from the US, which unilaterally withdrew from the deal last year.