The United States says a US Navy ship has "destroyed" an Iranian drone in the Strait of Hormuz after it threatened the vessel, a claim Iran promptly denied.
In remarks at the White House on Thursday, US President Donald Trump said the drone had flown to within 1,000 metres of the USS Boxer and had ignored "multiple calls to stand down".
"This is the latest of many provocative and hostile actions by Iran against vessels operating in international waters. The US reserves the right to defend our personnel, facilities and interests," Trump said.
"The drone was immediately destroyed," he added.
Abbas Araghchi, Iranian deputy foreign minister, dismissed the claim in a Twitter post on Friday.
"We have not lost any drone in the Strait of Hormuz nor anywhere else. I am worried that USS Boxer has shot down their own UAS [Unmanned Aerial System] by mistake!" he wrote.
The Pentagon said in a statement that the USS Boxer, an amphibious assault ship, had taken "defensive action" against an Iranian drone at 10am local time (07:00 GMT) on Thursday.
The incident took place in international waters while the ship was transiting the Strait of Hormuz to enter the Gulf, it said.
The Boxer is among several US Navy ships in the area, including the USS Abraham Lincoln, an aircraft carrier that has been operating in the nearby North Arabian Sea for weeks.
Tit for tat
Tensions in the Gulf are high, with fears that the US and Iran could stumble into war. Washington has blamed Tehran for a series of attacks since mid-May on shipping around the Strait of Hormuz, the world's most important oil artery. Tehran rejects the allegations.
Iran in June shot down a US military surveillance drone in the Gulf with a surface-to-air missile. Iran said the drone was in its airspace, but Washington said it was in international skies.
At the time, Trump said he called off a retaliatory military attack on Iran at the last moment.
Mark Fitzpatrick, associate fellow at the US-based International Institute for Strategic Studies, said each of the incidents increases the potential for conflict.
"None of the incidents have resulted in casualties so far. The drones were unmanned in a way that was tit for tat," he told Al Jazeera from Washington.
"They need to start talking to stop this escalatory cycle. Donald Trump himself has been saying he does want to talk with Iranians. So what exactly is the US strategy? No one in Washington knows."
Relations between the US and Iran have worsened since last year when Trump abandoned a 2015 deal between world powers and Iran in which Tehran agreed to curb its nuclear programme in return for the lifting of sanctions.
Since then, Washington has reinstated sanctions to throttle Tehran's trade, saying it wants to renegotiate the accord and hold talks on Iran's ballistic missiles programme and support for armed groups in the Middle East.
"The president wants to talk, but those around him don't want him to talk until the Iranians suffer more economically and they are trying to apply more pressure to make that happen," said Fitzpatrick.
Speaking to US-based media in New York, Zarif said Iran could immediately ratify a document prescribing more intrusive inspections of its nuclear programme if the US abandoned its economic sanctions.
The document, known as the Additional Protocol, gives UN inspectors from the International Atomic Energy Agency (IAEA) more tools to verify that a nuclear programme is peaceful.
Zarif blamed Washington for the escalation of tensions and accused the Trump administration of "trying to starve our people" and "deplete our treasury" through economic sanctions.
"We live in a very dangerous environment," he said. "The US has pushed itself and the rest of the world into probably the brink of an abyss."
Earlier on Thursday, Washington demanded Iran immediately release a vessel it seized in the Gulf, and a US military commander in the region said the US would work "aggressively" to ensure free passage of vessels through the vital waterway.
Iran's Revolutionary Guard Corps seized the foreign tanker accused of smuggling oil with a crew of 12 on Sunday, Iranian state TV reported.