Iran on Friday dismissed as "baseless" US accusations that it carried out twin attacks that left two tankers ablaze in the Gulf of Oman, escalating tensions across the region and sending world oil prices soaring.

US Secretary of State Mike Pompeo warned that Washington would defend its forces and allies in the region, and the United States pressed its case as the UN Security Council met to address the incident — the second in a month in the strategic shipping lane.

Foreign Minister Mohammad Javad Zarif tweeted on Friday that the US administration had "immediately jumped to make allegations against Iran without a shred of factual or circumstantial evidence".

He accused it of seeking to "sabotage diplomacy" amid a visit to Iran by Japanese Prime Minister Shinzo Abe and "cover up its economic terrorism against Iran" in enforcing crippling unilateral sanctions.

With tensions spiralling between Iran and the United States, the European Union called for "maximum restraint" and UN chief Antonio Guterres warned the world cannot afford a major confrontation in the Gulf.

Iran labelled the apparent attacks "suspicious" as its supreme leader Ayatollah Ali Khamenei rebuffed overtures by Abe to open talks with US President Donald Trump.

Pompeo said there was strong evidence of Iran's culpability, after the US Navy said it had spotted an unexploded limpet mine stuck to the hull of one of the vessels.

The US Central Command (CENTCOM) released a grainy black-and-white video it said showed an Iranian patrol boat "removing the unexploded limpet mine" from the ship.

"It is the assessment of the United States that the Islamic Republic of Iran is responsible for the attacks," Pompeo announced.

"This is based on intelligence, the weapons used, the level of expertise needed to execute the operation, recent similar Iranian attacks on shipping, and the fact that no proxy group operating in the area has the resources and proficiency to act with such a high degree of sophistication," he said.

Explosions off Iran

The two vessels were struck by explosions in the early daylight hours Thursday after passing through the Strait of Hormuz and travelling around 25 nautical miles off Iran's southern coast headed towards Asia.

The Norwegian-owned Front Altair ethanol tanker was hit by three explosions, according to the Norwegian Maritime Authority, and remained ablaze late Thursday.

Explosions also struck the Japanese-owned Kokuka Courageous, which was loaded with methanol, but the fire on board was soon extinguished. One crew member suffered minor injuries. The ship was on Friday heading towards the Omani port of Khor Fakkan.

There was no claim of responsibility for the blasts, which struck both tankers at the waterline.

Iran said its navy rescued several dozen crew members of the two vessels, while the US Navy said it had picked up 21 from the Kokuka Courageous.

Washington has dispatched the destroyer USS Mason to the scene "to provide assistance," CENTCOM said in a statement.

'Iran or proxies'

Pompeo called Thursday's tanker explosions "the latest in a series of attacks" he alleged were undertaken by Iran or its "proxies", including a Yemeni missile rebel missile which wounded 26 civilians at a Saudi airport on Wednesday.

A Saudi-led coalition which has been fighting the rebels it accuses of being Iranian proxies said Saudi air defences had intercepted a new rebel attack on a Saudi airport on Friday.

The abortive attack using five rebel drones targeted the southwestern city of Khamis Mushait, home to a huge airbase which has been the main launchpad of the coalition's more than four-year bombing campaign in Yemen.

"Taken as a whole, these unprovoked attacks present a clear threat to international peace and security," Pompeo said.

The United States has also accused Iran over May 12 attacks on four tankers anchored in the Gulf of Oman off the United Arab Emirates port of Fujairah.

A five-nation investigation into those attacks said its preliminary findings indicated a state actor was responsible but stopped short of naming Iran.

The US called on Thursday for the UN Security Council to confront the "clear threat" posed by Iran, as the council met behind closed doors to hear a briefing on Washington's assessment that Tehran was responsible.

Iran's mission to the UN pushed back, saying: "The US and its regional allies must stop warmongering."

Oil prices jumped at the threat of open conflict around the Strait of Hormuz, the chokepoint between the Gulf and the Gulf of Oman, through which transit some 15 million barrels per day of crude passes.

In London, benchmark Brent crude jumped 2.2 percent to $61.31 a barrel, while in New York, the US standard, West Texas Intermediate, rose 2.2 percent, to $52.28.

"We are in a dangerous moment in the region with this emerging pattern of attacks," said Elizabeth Dickinson, senior analyst with International Crisis Group.

"Any miscalculation or misunderstanding risks a spiral toward more direct confrontation."