Tensions in the Gulf region mounted Friday as Washington said an Iranian drone was destroyed after threatening an American naval vessel at the entrance of the Strait of Hormuz.

It was believed to be the first US military engagement with Iran following a series of increasingly serious incidents.

The USS Boxer, an amphibious assault ship, "took defensive action" against the Iranian drone on Thursday to "ensure the safety of the ship and its crew," Pentagon spokesman Jonathan Hoffman said.

The drone was "immediately destroyed" after it approached within 914 meters of the Boxer, US President Donald Trump told reporters.

"This is the latest of many provocative and hostile actions by Iran, against vessels operating in international waters," he added.

"The United States reserves the right to defend our personnel, our facilities and interests and calls upon all nations to condemn Iran's attempts to disrupt freedom of navigation and global commerce."

But Iran denied losing any drones. Armed forces spokesman Brigadier General Abolfazl Shekarchi described Trump's statement as "baseless and delusional claims," according to the Tasnim news agency.

"There have been no reports of a confrontation with the American USS Boxer," he was quoted as saying by Tasnim.

"Despite Trump's baseless and delusional claims, all of (Iran's) drones... have safely returned to their bases," said Shekarchi.

Deputy foreign minister Abbas Araghchi tweeted: "I am worried that USS Boxer has shot down their own (drone) by mistake!"

The confrontation came as Iran defended its detention Sunday of a "foreign tanker" believed to be the Panamanian-flagged vessel Riah for allegedly smuggling fuel.

"The vessel was en route to deliver contraband fuel received from Iranian boats to foreign ships in farther regions when it was intercepted," Iran's Islamic Revolutionary Guard Corps said on its Sepah News website.

Tehran downed an American surveillance drone in June, prompting Trump to order air strikes against Iran. But he called them off at the last minute, saying the plan was not "proportionate."

On July 4, Britain seized a fully loaded Iranian oil tanker near Gibraltar, allegedly headed for Syria in violation of sanctions.

Iran said the seizure was piracy, and one week later, armed Iranian boats threatened a British tanker in the Gulf before they were driven off by a Royal Navy frigate.

That led to US calls for an international flotilla to escort hydrocarbon-laden vessels from Gulf oil fields through the Strait of Hormuz.

The incidents have raised fears of a regional conflict involving the US and its allies in the Gulf region, through which nearly a third of the world's oil is transported.