A US Navy ship shot down an Iranian drone in the Strait of Hormuz after its operators ignored warnings to stand down, US President Donald Trump said on Thursday.
It is the first military action that Washington has taken against Tehran after a series of incidents in the Gulf where it has vowed to "aggressively" ensure the free passage of vessels.
The drone came within 1,000 metres of the USS Boxer, an amphibious assault ship, and the US took defensive action, Mr Trump said.
“The drone was immediately destroyed,” he said from the White House. “This is the latest of many provocative and hostile actions by Iran against vessels operating in international waters.
"The United States reserves the right to defend our personnel, facilities and interests."
The downing was preceded by an Iranian helicopter flying alongside the Boxer, yards away from the deck, according to the Wall Street Journal, which is aboard the vessel. Aircraft should not fly that close to vessels without their permission, it reported.
On board the USS Boxer in the Arabian Sea
The clash in one of the busiest waterways for international oil traffic highlighted the risk of conflict between two countries at odds over a wide range of issues. After Mr Trump pulled the United States out of the Iran nuclear deal last year and imposed additional economic sanctions, the Iranians have pushed back on the military front, allegedly sabotaging Saudi and other oil tankers in the Gulf, shooting down a US drone on June 20 and stepping up support for Houthi rebels in Yemen.
Mr Trump ordered a military strike in retaliation to the downing of the drone but called it off, saying that the casualty count would have been a disproportionate reaction.
Adding to the economic pressure on Tehran, the Treasury Department said Thursday it was imposing sanctions on what it called a network of front companies and agents involved in helping Iran buy sensitive materials for its nuclear program. It said the targeted individuals and entities are based in Iran, China and Belgium.
But the downing of the drone marked a new escalation between Washington and Tehran. Chief Pentagon spokesman Jonathan Hoffman confirmed the strike against the drone on Thursday.
"At approximately 10am local time, the amphibious ship USS Boxer was in international waters conducting a planned inbound transit of the Strait of Hormuz," Mr Hoffman said.
"A fixed-wing unmanned aerial system approached Boxer and closed within a threatening range. The ship took defensive action to ensure the safety of the ship and its crew."
After the drone was allegedly shot down, Iranian Foreign Minister Javad Zarif tweeted the word "reminder", with a map showing how far the US is from Iran and Gulf waters.
He later told reporters at the United Nations: "We have no information about losing a drone today."
Iran's deputy foreign minister Abbas Araghchi denied on Friday having lost any drone recently and hinted that the US could have downed their own "by mistake."
Iranian leaders have earlier called on the US to withdraw from the area.
In an apparent response to the Iranian attacks in the Gulf, Britain seized a fully loaded Iranian oil tanker near Gibraltar on July 4, allegedly headed for Syria in violation of sanctions.
Gibraltar's Supreme Court ruled on Friday that The Grace 1 supertanker, carrying 2.1 million barrels of oil can be detained for 30 more days, the British territory's attorney general said.
Iran furiously branded the seizure "piracy," and one week later armed Iranian boats menaced a British tanker in the Gulf, before they were driven off by a Royal Navy frigate.
Earlier on Thursday, the United States demanded Iran immediately release a vessel it seized in the Gulf.
Iran played down the seizure of the ship, which it said was a small vessel that was smuggling oil.
Iranian state TV aired footage of a vessel called "RIAH." The Panamanian-flagged oil tanker MT Riah disappeared off trackers in Iranian territorial waters days ago.
US Central Command chief Kenneth McKenzie pledged Thursday to work "aggressively" with partners to ensure freedom of navigation in Gulf waters.
US Special representative for Iran Brian Hook will meet ambassadors at the State Department on Friday to promote an American plan to build a maritime coalition to deter further attacks in the Gulf.
Mr Hook said the Sentinel Programme “will make it harder for Iran to disrupt freedom of commerce and navigation”.
He did not specify which countries would attend the meeting and it is unclear which states have agreed to take part.
The US also plans to install surveillance and protection equipment on ships travelling through the Gulf.
Earlier on Thursday, the US Navy announced that it had launched a search for a missing sailor in an “overboard incident” in the Arabian Sea.
Washington has recently beefed up its military presence and the U.S. Fifth Fleet, based in Bahrain, says that Gulf Arab states have stepped up patrols.
Islamic Revolutionary Guards Corps Commander-in-Chief Hossein Salami said on Thursday that Iran had adopted a defensive strategy, but warned that "if our enemies make any mistakes ... our strategy can become an offensive one."
Behind the Gulf tensions lay a fractious dispute over the 2015 nuclear deal signed between Tehran and world powers. Washington says Iran was violating the deal before it started reducing its commitments to the pact, which aimed to curb its uranium enrichment in return for the lifting of sanctions, in May. The European signatories of the deal, Britain, France and Germany, have criticised Iran for not upholding its commitments.
But Iran on Thursday signaled a willingness to engage in diplomacy to defuse tensions with the United States with a modest offer on its nuclear program that met immediate scepticism in Washington.
Iran's foreign minister told reporters in New York that Iran could immediately ratify a document prescribing more intrusive inspections of its nuclear program if the United States abandoned its economic sanctions.
Washington is trying to force Tehran to agree to stricter limits on its nuclear capacity, curb its ballistic missile program and end support for proxy forces in Yemen, Syria and Lebanon.