two tankers were targeted in suspected attacks near the strategic Strait of Hormuz. (AFP)

The perilous contest between the United States and Iran for influence in the Middle East escalated dramatically on Thursday as two tankers were targeted in suspected attacks near the strategic Strait of Hormuz, a vital waterway for global oil shipments.

Secretary of State Mike Pompeo blamed Iran for the "blatant assault" on the vessels and said the United States would defend itself and its allies against Iranian aggression in the region. But he provided no evidence that the explosions had been the work of Iranian forces.

The blasts appeared to be a coordinated attack, damaging the hull of a Japanese-owned tanker and striking another Norwegian-owned vessel, which caught fire and was left adrift in the Gulf of Oman.

The attacks were similar to suspected acts of sabotage carried out against tankers near the United Arab Emirates' Fujairah port last month and looked to be the latest salvo in the mounting confrontation between the U.S. and Iran. As the Trump administration has tightened economic sanctions on Iran after withdrawing last year from the historic nuclear deal, Iran and its allies have responded with a series of calibrated attacks in the Gulf, Iraq and Saudi Arabia aimed at underscoring the potential cost to U.S. interests, including the international oil trade, experts say.

Iranian Foreign Minister Mohammad Javad Zarif described the incidents Thursday as "suspicious," and Iran's naval forces said they were investigating the "accident" in the Gulf of Oman.

The crews of both vessels were force to abandon ship. A U.S. defense officials said the USS Bainbridge, an Arleigh Burke-class guided missile destroyer that was in the area, took on board 21 crew members from the Japanese vessel, the Kokuka Courageous.

The Gulf of Oman has been a flash point for tensions between Iran, the United States and its Arab gulf allies. (AFP)

The attack on the Japanese ship appeared timed to undermine diplomatic efforts by Japan's prime minister, Shinzo Abe, who was wrapping up a high-stakes visit to Tehran. He met there with Iranian supreme leader, Ayatollah Ali Khamenei, and was seeking to help mediate potential talks between U.S. and Iranian officials. Abe had called his talks in Tehran "a major step forward toward securing peace and stability in this region," the Kyodo news agency reported.

The second vessel, owned by Norway's Frontline, was "suspected of being hit by a torpedo," an official with Taiwan's state oil refiner, CPC Corp., which chartered the vessel, told the Reuters news agency.

The tanker, called the Front Altair, was carrying naphtha, a flammable petrochemical product that was loaded at a port in the United Arab Emirates and destined for East Asia, news agencies reported. The ship's 23 crew members - 11 Russians, 11 Filipinos and one Georgian - were rescued by a nearby vessel and transferred to an Iranian navy ship, then taken to the Iranian port of Bandar Abbas.

Speaking to reporters, Pompeo said the U.S. assessment of Iranian involvement is based on intelligence, the type of weapons used and the level of expertise needed, and that no other Iranian-backed militia in the region has the resources or proficiency to pull off such a sophisticated operation.

He said the impetus behind the attacks was the administration's "maximum pressure campaign" of sanctions that U.S. officials say are designed to get Iran to negotiate over its nuclear program and its support of militias in various neighboring countries.

"Our policy remains an economic and diplomatic effort to bring Iran back to the negotiating table at the right time and encourage a comprehensive deal that addresses the broad range of threats," Pompeo said. "Iran should meet diplomacy with diplomacy, not with terror, bloodshed and extortion."

Shortly after Pompeo spoke, senior U.S. officials showed photographs to reporters of the damaged tanker "Courageous" with what the Navy identified as a suspected magnetic mine stuck to its hull. The unexploded weapon was probably applied by hand from an Iranian fast boat, one official said, and is thought to be the same kind of weapon used to blow a hole elsewhere in the same tanker and to do more serious damage to the other ship, the Front Altair, two officials said.

The officials, who briefed reporters on condition of anonymity because many elements of the investigation remain secret, said the type and timing of the attack bear Iranian hallmarks, but U.S. officials cannot yet say for sure where the mines were manufactured or exactly how they were laid.

"There's not too many ways in which this can be done," one official said. "Very few that don't involve an individual physically placing it on the ship."

U.S. officials said several nations are consulting about how to respond. One option may be to provide military escorts for commercial tankers moving through the strait, one official said, although no decision has been made.

"As the threat evolves it's incumbent on us to re-evaluate our presence," said one senior U.S. official.

The U.S. military has already dispatched a P-8 Poseidon, an anti-ship, anti-submarine and surveillance aircraft, to the area in response to the incident, a defense officials said.

Senior U.S. officials have been in frequent touch about the incident since Thursday morning, the official added. U.S. Central Command is sending additional troops and weaponry to the Middle East.

The Gulf of Oman links the Arabian Sea with the Strait of Hormuz, the gateway to the Persian Gulf. The gulf has been a flash point for tensions between Iran, the United States and its Arab gulf allies.

"The tension in the Strait of Hormuz and the Persian Gulf is now as high as it gets without being an actual armed conflict," said Jakob Larsen, head of maritime security at the Baltic and International Maritime Council (Bimco), the world's largest international shipping association. "The increase in attacks and the escalated threat to seafarers is an urgent concern to the industry."

U.S. officials say the administration's pressure campaign is aimed at isolating Tehran, curb its support for regional proxies and advanced ballistic missile program, and ultimately bring Tehran back to the negotiating table. But some experts say the recent tensions have underscored the limits of that policy.

In a climate of hostility, the tanker incidents could bring the parties closer to the brink of violent confrontation.

"This is a way station to a wider conflict breaking out between Iran and the United States," said Ali Vaez, senior Iran analyst and Iran project director for the International Crisis Group. "If Iran was behind it, it is very clear the maximum pressure policy of the Trump administration is rendering Iran more aggressive, not less."

The attacks could also reflect a widening split between pro-diplomacy officials in Iran and hardliners opposed to further negotiations, including the powerful Islamic Revolutionary Guard Corps, or IRGC. The paramilitary organization, which boasts land, air and sea forces, answers only to Khamenei and is responsible for Iran's external military operations.

Iran's security services, including the IRGC, "have a decades-long history of conducting attacks and other operations aimed precisely at undermining the diplomatic objectives of a country's elected representatives," the political risk firm Eurasia Group said in a briefing note Thursday.

"The attacks could have been designed to put an exclamation point on Iran's warnings to Abe about the risks of instability in the region," the note said. About 80 percent of Japan's oil imports come from the Middle East and travel through the Strait of Hormuz.

The attacks occurred 24 nautical miles from the nearest IRGC naval base, one U.S. official said. IRGC ships are frequently present in that area, but had not until recently begun to harass or impede shipping, the official said.

"It's clear that there is a pattern of Iranian naval activity in and around commercial shipping lanes that is inconsistent with their prior behavior," the official said.

The attacks are part of Iran's response to tightening U.S. sanctions, one official said. He described the Iranian view this way: "If we can't ship oil, no one can."