Two oil tankers near the strategic Strait of Hormuz were damaged Thursday in suspected attacks that left one ablaze and the other adrift, forcing sailors to evacuate in the latest mysterious incident involving tankers amid heightened tensions between the United States and Iran.

The U.S. Navy, which rushed in to assist the stricken vessels, and the ship's owners offered no immediate explanation on what weapon caused the damage to the MT Front Altair and the Kokuka Courageous in the Gulf of Oman off the coast of Iran, though all believed the ships had been targeted in an attack. The Front Altair burned for hours afterward, charring half of one of the vessel's sides and sending thick black smoke into the sky.

Last month, the U.S. alleged Iran used limpet mines to attack four oil tankers off the nearby Emirati port of Fujairah. Iran denied being involved, but the incidents came as Iranian-backed rebels in Yemen also have launched missile and drone attacks on Saudi Arabia, including on oil infrastructure.

Meanwhile in Iran, Supreme Leader Ayatollah Ali Khamenei warned that while Tehran doesn't seek nuclear weapons, "America could not do anything" to stop Iran if it did.

The comments came during a one-on-one meeting capping Japanese Prime Minister Shinzo Abe's high-stakes visit in Tehran that sought to ease Iran-U.S. tensions, suggesting the efforts had failed.

Benchmark Brent crude spiked by as much as 4% at one point in trading following the suspected attack, to over $62 a barrel, highlighting how crucial the area remains to global energy supplies. A third of all oil traded by sea passes through the strait, which is the narrow mouth of the Persian Gulf.

Cmdr. Joshua Frey, a 5th Fleet spokesman, said the U.S. Navy was assisting the two vessels that he described as being hit in a "reported attack." He did not say how the ships were attacked or who was suspected of being behind the assault. The American destroyer USS Bainbridge was sent to assist, he said.

The MT Front Altair, a Marshall Islands-flagged crude oil tanker, immediately caught fire after the suspected attack.

The firm that operates the Front Altair told The Associated Press that an explosion was the cause of the fire onboard. The crew of 23 is safe after being evacuated by 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 have been transferred to an Iranian port in the southern province of Hormozgan.

The Front Altair came from Ruwais, United Arab Emirates, a loading point for the state-run Abu Dhabi National Oil Co., according to the data firm Refinitiv. It carried naphtha, a flammable hydrocarbon. The Kokuka Courageous, came from Mesaieed, Qatar; and Jubail, Saudi Arabia, carrying methanol, a chemical compound used in a variety of products, Refinitiv said.

The fact that both vessels remained afloat suggested mines may have damaged them, said Jakob P. Larsen, head of maritime security for BIMCO, the largest international association representing ship owners.

"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.

He added, "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."

The timing of Thursday's suspected attack was especially sensitive as Mr. Abe's high-stakes diplomacy mission was underway in Iran. Japan's Trade Ministry said the two vessels had "Japan-related cargo," without elaborating.

On Wednesday, after talks with Iranian President Hassan Rouhani, Mr. Abe warned that any "accidental conflict" that could be sparked amid the heightened U.S.-Iran tensions must be avoided.

His message came just hours after Yemen's Iranian-backed Houthi rebels attacked a Saudi airport, striking its arrivals hall before dawn and wounding 26 people Wednesday. A Saudi-led coalition has been fighting for over four years against the Houthis, whom United Nations experts and the West say receive arms from Iran. Tehran denies that.

A statement published by Mr. Khamenei's website after Mr. Abe's meeting Thursday with the supreme leader suggested a tense exchange between the two.

"We have no doubt about your good will and seriousness, but ... I don't regard [President Donald] Trump as deserving any exchange of messages," Mr. Khamenei reportedly told Mr. Abe.

Mr. Khamenei also said Iran remained opposed to building atomic weapons, but he offered a challenge to Mr. Trump.

"You should know that if we planned to produce nuclear weapons, America could not do anything," said Mr. Khamenei, who has final say on all matters of state in Iran's Shiite theocracy.

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 pulled out of last year.

Iran's nuclear deal saw Tehran agree to limit its enrichment of uranium in exchange for the lifting of crippling sanctions.

In withdrawing from the deal last year, Mr. Trump pointed to the accord not limiting Iran's ballistic missile program and not addressing what American officials describe as Tehran's malign influence across the wider Middle East. Those who struck the deal at the time described it as a building block toward further negotiations with Iran, whose Islamic government has had a tense relationship with America since the 1979 takeover of the U.S. Embassy in Tehran and subsequent hostage crisis.

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Already, Iran says it quadrupled its production of low-enriched uranium. Meanwhile, U.S. 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.

This story was reported by The Associated Press. AP writers Aya Batrawy in Dubai, David Rising in Berlin, Mari Yamaguchi in Tokyo, and Amir Vahdat in Tehran, Iran, contributed to this report.