TOKYO — Japanese Prime Minister Shinzo Abe asked Iran’s leaders to release several U.S. citizens being held in that country during his visit to Tehran this week, the official Kyodo News agency reported Friday, citing a government official. The Iranian response is not known.

At least five American citizens have been imprisoned or are awaiting trial in Iran.

Abe met Iranian leaders on Wednesday and Thursday in a high-stakes attempt to defuse rising tensions between Iran and the United States. But his task was complicated Thursday by suspected attacks on two tankers, including one owned by a Japanese shipping company, in the Gulf of Oman.

Abe described his meeting Thursday with Iran’s supreme leader, Ayatollah Ali Khamenei, as “a major step forward toward securing peace and stability in this region,” Kyodo News reported, adding that Khamenei had conveyed his “belief in peace.”

On Friday, Japanese Foreign Minister Taro Kono described Abe’s meeting as “fruitful,” noting that Khamenei said his country had no intention of producing nuclear weapons.

But Khamenei was also quoted by Iranian state media as saying that President Trump is not “worthy” of exchanging messages with, and that Iran would not repeat its “bitter experience” of negotiating with the United States.

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. The Iranian mission to the United Nations denied any involvement and called the U.S. claim “inflammatory” and “Iranophobic.”

Iranian Foreign Minister Mohammad Javad Zarif also tweeted that the fact American officials immediately blamed Iran, without “a shred of factual or circumstantial evidence,” makes clear the United States is moving to what he called “sabotage diplomacy” to cover up its “economic terrorism” against Iran.

He expressly included Abe in this alleged conspiracy, underlining the gulf of mistrust the Japanese leader may have had to bridge in Tehran as a U.S. ally.

Nevertheless, Japan’s chief cabinet secretary, Yoshihide Suga, put a positive spin on the meetings.

“The path toward easing of the tension will come with challenges,” he told reporters on Friday. “On its part, Japan will continue to make efforts to achieve peace and stability in the region.”

About 80 percent of Japan’s oil imports come from the Middle East and travel through the Strait of Hormuz.

Japan is a key U.S. ally but also enjoys long-standing diplomatic and cultural ties with Iran. It opposed Trump’s decision to pull out of the 2015 Iran nuclear agreement negotiated under the Obama administration between Tehran and six world powers.

Japanese media have largely supported Abe’s attempt to defuse tensions, but the timing of the suspected attacks was embarrassing for the Japanese leader. 

“It is an awkward ‘coincidence’ and suggests that Abe’s visit is little more than a photo op with zero impact on tensions,” Jeff Kingston, a professor at Temple University’s Japan Campus in Tokyo, wrote in an email. “Whatever one thinks about Khamenei’s belief in peace, actions speak louder and leave Abe with egg on his face, stumbling into a volatile situation where he looks like an amateur in over his head.”

But Michael Bosack, a special adviser at the Yokosuka Council on Asia-Pacific Studies in Japan, said it was too early to say who had carried out the attack or whether Japan was targeted.

“In the meantime, this type of incident reinforces the importance of Abe’s efforts to mediate tensions in the region,” he said, noting Japan’s dependence on the Strait of Hormuz for its energy needs. “Japan has a stake in the game for seeing an end to these incidents and a peaceful resolution to the destabilizing tensions between the United States and Iran.”

Abe, who first visited Iran with his father in 1983, has maintained close ties with the Iranian leadership since becoming prime minister. The two countries signed an investment agreement in 2016 and are celebrating 90 years of diplomatic engagement this year.

“This is only the initial phase of Abe’s shuttle diplomacy, but he’s executed his role well thus far,” said Bosack. “Now it will be up to him to reengage with the White House, where we’ll see if his efforts can bear some fruit.”

Akiko Kashiwagi contributed to this report.