TEHRAN/TOKYO -- Two oil tankers, one belonging to a Japanese shipping company, were attacked in Oman Bay near the Strait of Hormuz on Thursday.
U.S. Secretary of State Mike Pompeo placed the blame on Iran, based on the degree of sophistication of the attack.
"It is the assessment of the United States government that the Islamic Republic of Iran is responsible for the attacks that occurred in the Gulf of Oman today," Pompeo told reporters in Washington on Thursday.
"This assessment 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," Pompeo said.
Japanese Prime Minister Shinzo Abe was visiting Iran at the time.
Resource-poor Japan imports nearly all its oil from the politically volatile Middle East, and Abe was in Iran to mediate a growing rift between Tehran and Washington.
The ships were the Panama-listed tanker Kokuka Courageous and the Marshall Islands-flagged Front Altair. The vessels were attacked at 11:45 a.m. Japan time, according to the Japanese government.
Kokuka Courageous is owned by Tokyo-based shipping company Kokuka Sangyo, which is 50% owned by Mitsubishi Gas Chemical. The vessel was carrying methanol.
Kokuka Sangyo President Yutaka Katada expressed outrage over the attack during a news conference in Tokyo late Thursday. "Why is our ship the one to be on the receiving end of the attacks?" Katada said. "I'm enraged that the lives of the crew members and the safety of the vessel were threatened."
The Kokuka Courageous sustained two attacks, the first occurring around 11:45 a.m. Thursday, Japan time. An area near the engine room, on the rear port side of the tanker, was hit by what is believed to be a shell. The flames were extinguished by crew members.
The second attack came roughly three hours later, striking the center of the ship on the port side. The 21 Filipino crew members decided that remaining on the ship was unsafe, and they left the vessel on lifeboats. The unmanned tanker remains adrift at sea, according to latest reports. In both attacks, the ship was hit just above the waterline.
The U.S. Navy's guided missile destroyer USS Bainbridge, which was operating in the vicinity, provided immediate assistance to the Kokuka Courageous, Central Command spokesman Lieutenant Colonel Earl Brown said.
"Twenty-one mariners from the M/V Kokuka Courageous, who abandoned ship, are currently aboard USS Bainbridge," Brown said.
The Strait of Hormuz is one of the world's most crucial sea lanes for oil shipments.
Last month, four Saudi Arabian vessels sustained attacks in the same region. Pompeo said both those and Thursday's attacks were part of "a series of attacks instigated by the Islamic Republic of Iran and its surrogates against American and allied interests."
Jonathan Cohen, acting U.S. ambassador to the United Nations, told a meeting of the Security Council on Thursday, that an attack on commercial shipping was "unacceptable."
"Today's attacks on ships in the Gulf of Oman raise very serious concerns," Cohen said.
The identity of the attackers is unknown. Pompeo did not provide explicit evidence of Iran's involvement and took no questions from reporters after reading a short statement.
"It's hard to believe that this came at the direction of the Iranian leadership, who have no intention of escalating tension with the U.S.," said Shuji Hosaka, senior research fellow at the Tokyo-based Institute of Energy Economics.
Hosaka noted that the attacks occurred during Abe's visit to Iran. "It may have come from forces that want to stop Iran-U. S. relations from improving," he said, suggesting an extremist group like Islamic State could be responsible.
Oil prices surged on reports of the attacks. Brent crude futures were up as much as 4.45% before retreating. U.S. West Texas Intermediate crude futures rose as much as 3.85% before similarly falling.
"The most immediate impact has been an increase in the oil price. The ramifications have not yet been felt on tanker charter rates, and they might not be if incidents remain isolated," said Adam Kent, managing director of Maritime Strategies International.
"We're likely to see further increases -- both for vessel and cargo insurance -- in light of today's event, which may ultimately be passed on to the end consumer," he said. "A number of tanker owners are already considering stopping future callings in the Middle East until the situation has been resolved."
Hiroshige Seko, minister of economy, trade and industry, on Thursday said Japan's energy supply has not been impacted.
Abe on Wednesday became the first Japanese leader to visit Tehran since the 1979 Iranian Revolution.
Atsushi Teraoka and Takumi Kitamoto in Tokyo and Marrian Zhou in New York contributed to this report.