President Donald Trump has said a US warship destroyed an Iranian drone in the Strait of Hormuz after it ignored warnings to back off.
He said the USS Boxer took "defensive action" after the drone ignored repeated warnings and came within 1,000 yards of the ship.
The president called it a "provocative and hostile" action by Iran.
The "fixed-wing unmanned aerial system... closed within a threatening range" at 10am local time on Thursday, according to a Pentagon statement.
"The ship took defensive action against the UAS to ensure the safety of the ship and its crew," it added.
However, Iran's foreign minister told reporters at the United Nations "we have no information about losing a drone today".
It comes amid a period of heightened tension between the two countries, which included Iran last month shooting down a US drone it said was flying over the country.
That incident saw Mr Trump call off a retaliatory strike at the last minute because he said too many people would have died.
The Strait of Hormuz is a vital shipping lane for oil and gas.
However, it has become a flash-point following the US decision to pull out of a nuclear deal with Iran and impose strict sanctions that are seriously damaging its economy.
A foreign tanker was seized by Iranian authorities in the strait on Sunday, according to state television.
It claimed the ship - which was not named but is believed to be the Panamanian-flagged MT Riah - was smuggling a million litres of oil out of the country.
Iran has recently increased uranium production and enrichment over the limits of the 2015 nuclear deal to try to put pressure on Europe to offer it better terms and allow it to sell its oil abroad.
The US has subsequently sent thousands of additional troops, nuclear-capable B-52 bombers and fighter jets to the region.
Western powers also blamed Iran for two mine attacks on oil tankers in the region last month - something it denies.
The UK also has strained relations with Iran after Royal Marines seized a tanker carrying around $120m of oil off Gibraltar.
The operation, at the start of July, was carried out over suspicions the ship was breaking EU sanctions by carrying Iranian oil to Syria.
Iran said the action was illegal and warned the UK it was playing a "dangerous game" with "consequences".
Britain is sending a third major warship to the region , with the frigate HMS Kent heading to the Gulf from mid-September.
However, the Ministry of Defence said it was part of a planned deployment and not related to the current tensions with Iran.
The UK also has four minehunters in the region.