US Secretary of State Mike Pompeo said that the Trump Administration blames Iran for attacks on two oil tankers near the Persian Gulf, casting it as the latest in a series of “provocative actions” that have sharply raised tensions in the region.

A US assessment of Iran’s responsibility, which forced the evacuation of the crews in international waters, was based in part on intelligence as well as the expertise needed to carry out the operation, Pompeo told reporters in Washington.

It was also based on a recent series of incidents in the region that the US also blames on Iran, including a similar attack on tankers in the area in May and the bombing of an oil pipeline in Saudi Arabia by Iranian-backed fighters, he said.

An Iranian navy boat sprays water to extinguish a fire on an oil tanker in the sea of Oman. (AP)

“Taken as a whole these unprovoked attacks present a clear threat to international peace and security, a blatant assault on the freedom of navigation and an unacceptable campaign of escalating tension by Iran,” Pompeo said.

The US planned to raise the attacks at the UN Security Council later Thursday. Pompeo also said the US would defend its forces and interests in the Middle East but gave no details on any immediate plans.

The US is sending a second warship to the area where the tankers were attacked.

US Central Command said the guided-missile destroyer USS Mason is joining US forces helping one of the merchant tankers.

There are unconfirmed reports a torpedo was used in the attack.

US Secretary of State Mike Pompeo has blamed Iran for the attacks on two oil tankers near the Persian Gulf. (AP)

Oil prices jumped 2 per cent after the attacks.

US West Texas Intermediate (WTI) crude futures were up $US1.14, or 2.23 per cent, at $US52.28 a barrel.

WTI earlier rose as much as 4.5 per cent to $US53.45.

There are concerns the price rise will lead to higher petrol prices throughout the world.

A third of all oil traded by sea passes through the strategically important Strait of Hormuz, the narrow mouth of the Persian Gulf near the Gulf of Oman.

The tankers near the strategic Strait of Hormuz came under a suspected attack, with one of the vessels set ablaze in a suspected assault.

(AP)

Details of the attack remained unclear but the crew of one of the ships, the Front Altair, reported reported hearing three explosions, reports the Telegraph.

The ship was “suspected of being hit by a torpedo”, according to Taiwan’s state-owned petrol company

The US Navy rushed in to assist the stricken vessels in the Gulf of Oman off the coast of Iran. The ship's operators offered no immediate explanation on what caused the damage or who used them against the MT Front Altair and the Kokuka Courageous.

Each was loaded with petroleum products, and the Front Altair burned for hours, a fire that charred half of one of the vessel's sides and sent up a column of thick, black smoke.

The oil tanker Front Altair seen through a glass observation window as water cannon sprays the stricken ship. (AP)

It's the latest mysterious incident targeting vessels in a region crucial to global energy supplies amid heightened tension between Iran and the US.

Last month, the US alleged that Iran used limpet mines to attack four oil tankers off the nearby Emirati port of Fujairah. Iran denied being involved then and its foreign minister called the timing of Thursday's incidents suspicious, given that Japanese Prime Minister Shinzo Abe was meeting Supreme Leader Ayatollah Ali Khamenei in Tehran.

An oil tanker is on fire in the sea of Oman. (AAP)

Iran previously used mines against oil tankers in 1987 and 1988 in the "Tanker War," which saw the US Navy escort ships through the region.

Regardless of who is responsible, the price of a barrel of benchmark Brent crude spiked as much as four per cent immediately after the attack, showing how critical the region remains to the global economy.

"We need to remember that some 30 per cent of the world's (seaborne) crude oil passes through the straits. If the waters are becoming unsafe, the supply to the entire Western world could be at risk," said Paolo d'Amico, chairman of INTERTANKO tanker association.

"The shipping industry views this as an escalation of the situation, and we are just about as close to a conflict without there being an actual armed conflict, so the tensions are very high," Jakob P. Larsen, head of maritime security for BIMCO, the largest international association representing ship owners, said.

Two oil tankers near the strategic Strait of Hormuz have been damaged in suspected attacks. (AAP)

The suspected attacks happened at dawn on Thursday some 40 kilometres off the southern coast of Iran.

The Norwegian-owned Front Altair, loaded with naptha from the United Arab Emirates, radioed for help as it caught fire. A short time later, the Japanese-owned Kokuka Courageous, loaded with methanol from Saudi Arabia and Qatar, also called for help.

The US Navy sent a destroyer, the USS Bainbridge, to assist, said Commander Joshua Frey, a 5th Fleet spokesman. He described the ships as being hit in a "reported attack," without elaborating.

US President Donald Trump "has been briefed on the attack," White House press secretary Sarah Sanders said.

"The US government is providing assistance and will continue to assess the situation."

Kokuka Sangyo President Yutaka Katada points to a picture of his company ship 'Kokuka Courageous'. (AAP)

Frontline, the firm that operates the Front Altair, told The Associated Press that an explosion was the cause of the fire. Its crew of 23 — from Russia, the Philippiines and Georgia — was safely evacuated to the nearby Hyundai Dubai vessel, it said.

BSM Ship Management said the Kokuka Courageous sustained hull damage and 21 sailors had been evacuated, with one suffering minor injuries.

Iranian state television said 44 sailors from the two tankers were transferred to an Iranian port in the southern province of Hormozgan. The Japanese owners of the Kokuka Courageous said, however, that its 21 Filipino sailors were aboard a US warship. The discrepancy could not be immediately reconciled.

The Front Altair had been bound for Taiwan, the Kokuka Courageous for Singapore, according to the to the data firm Refinitiv.

The fact that both vessels remained afloat suggested mines may have damaged them, Larsen said.

"Judging from the damage, it doesn't look like a torpedo, it looks like something smaller, whether that is a vessel-borne improvised explosive device or even a limpet mine," he told the AP.

Like in Fujairah, dozens of ships ranging from massive oil tankers to smaller pleasure boats, traditional dhows and cargo vessels ply the waters of the strait and the Gulf of Oman. The navies of Iran, Oman, the UAE and the US regularly patrol, but the waters are vast and lit only by the moonlight at night, allowing small vessels to approach without warning.

Tensions have escalated in the Mideast as Iran appears poised to break the 2015 nuclear deal with world powers, an accord that the Trump administration repudiated last year. The deal saw Tehran agree to limit its enrichment of uranium in exchange for the lifting of crippling sanctions. Now, Iran is threatening to resume enriching uranium closer to weapons-grade levels if European nations don't offer it new terms to the deal by July 7.

Japanese Prime Minister Shinzo Abe is in Iran. (AAP)

Already, Iran says it quadrupled its production of low-enriched uranium. Meanwhile, US sanctions have cut off opportunities for Iran to trade its excess uranium and heavy water abroad, putting Tehran on course to violate terms of the nuclear deal regardless.

Thursday's incidents came as Abe's high-stakes diplomatic mission was underway in Iran. Japan's Trade Ministry said the two vessels had "Japan-related cargo," without elaborating.