Iranian vessels unsuccessfully tried to impede the passage of a British commercial vessel through the Strait of Hormuz, the UK's government said, an accusation that Iran denied.
In a statement on Thursday, the UK said the Iranian ships on Wednesday only turned away after receiving "verbal warnings" from a UK navy vessel accompanying the commercial vessel British Heritage.
"We are concerned by this action and continue to urge the Iranian authorities to de-escalate the situation in the region," the statement said.
Iran's Foreign Minister Mohammad Javad Zarif rejected the accusation.
"Apparently the British tanker has passed. What they have said themselves and the claims that have been made are for creating tension and these claims have no value," he said.
The Iranian Revolutionary Guard Corps (IRGC) also denied involvement, saying if it had received orders to seize any ships it would have executed them immediately.
"There were no clashes with alien boats, especially English boats," IRGC said in a statement.
Earlier on Thursday, two US officials told Reuters news agency that five boats believed to belong to the Guard were involved in the incident.
"The Royal Navy HMS Montrose, which was also there, pointed its guns at the boats and warned them over radio, at which point they dispersed," one official with knowledge of the incident said.
"It was harassment and an attempt to interfere with the passage," the other official said.
The US military confirmed the incident in a statement but did not share any further details.
The incident came a day after Iran's President Hassan Rouhani warned of repercussions for the seizure of its own supertanker off the UK territory of Gibraltar last week.
About 20 percent of all oil traded worldwide passes through the Strait of Hormuz.
Threats to international freedom of navigation require an international solution, US Central Command said.
"The world economy depends on the free flow of commerce, and it is incumbent on all nations to protect and preserve this lynchpin of global prosperity," Captain Bill Urban, a spokesman for US Central Command, said in a statement.
China called on all sides to avoid raising already-high tensions in the Gulf. Foreign ministry spokesman Geng Shuang said Beijing hopes the parties involved can "maintain cool and restraint" and safeguard peace and stability in the region.
Russian government spokesman Dmitry Peskov told reporters "freedom of navigation should be ensured" in the Gulf and Strait of Hormuz, which he said is crucial for the global economy. Peskov called on all parties to settle their disputes through negotiations.
The British government said the tanker it seized was believed to be carrying two million barrels of Iranian crude oil to Syria in breach of European Union sanctions against the government of Syrian President Bashar al-Assad.
Tehran denied the tanker was bound for Syria and called its seizure "piracy". It also summoned the British envoy in Iran to protest against the action.
On Thursday, a spokesman for the Royal Gibraltar Police said they arrested the captain and chief officer of the supertanker carrying the Iranian oil.
Patrick Payas told The Associated Press the two are in police custody while investigations continue into the movements of the Grace 1 supertanker. He said they have not been formally charged.
Tensions between Iran and the US and its allies have risen sharply since Washington last year unilaterally withdrew from a landmark nuclear deal signed between Tehran and world powers in 2015.
Since then, the administration of US President Donald Trump has stepped up economic sanctions against Iran and moved to bring the country's oil exports to zero as part of a "maximum pressure" policy to make it halt actions that it said undermined regional security.
Iran has responded to the sanctions by starting to exceed limits put on its nuclear activities under the 2015 deal.
Trump, who has said he will not allow Tehran to acquire nuclear weapons, said on Wednesday that "Iran has long been secretly enriching uranium".
Tehran, which denies seeking nuclear weapons, said its latest measures were within the framework of the deal and dubbed the US sanctions "economic terrorism".
Several oil tankers were attacked in waters near Iran's southern coast in May and June, for which the US blamed Iran. Tehran denied any involvement.
Last month, Iran shot down a US drone near the Strait of Hormuz, prompting Trump to order retaliatory air raid, only to call them off at the last minute.
On Tuesday, the US said it was working to form a military coalition to safeguard strategic waters off Iran and Yemen.
Squeezed by sanctions, Iran has delivered threats to disrupt oil flow through the strategic Strait. Last year, an IRGC commander had threatened to block all exports through the Strait if their exports were stopped.