Japan had previously said that one Japanese citizen was arrested on January 20, with reports saying he was the Tehran bureau chief of public broadcaster NHK.

The identity of the second Japanese person held and the date as well as other details of their arrest were unclear.

News of the additional detention comes after the United States and Israel began a sweeping military campaign against Iran that has resulted in Tehran firing retaliatory strikes around the region.

"The government will continue to strongly press for an early release and, while staying in contact with the individuals, their families and all parties concerned, will provide every possible form of support," Foreign Minister Toshimitsu Motegi said, Kyodo News reported.

A spokeswoman for the Japanese foreign ministry told AFP -- without naming either person -- that embassy staff have been in contact with the two people and to "confirm their safety".

According to Radio Free Europe/Radio Liberty's Radio Farda and the Committee to Protect Journalists (CPJ), one of those arrested is the Tehran bureau chief of Japanese public broadcaster NHK, Shinnosuke Kawashima.

Kawashima was arrested on January 20 by the Islamic Revolutionary Guard Corps (IRGC), the CPJ said on February 26, citing a source who spoke on condition of anonymity for fear of retaliation.

The CPJ and Radio Free Europe both reported that Kawashima was transferred on February 23 to Evin Prison, where political prisoners are usually incarcerated.

The CPJ's source also said that NHK's longtime videographer Mehdi Mohammedi's passport was confiscated, along with his personal devices, including his phone, laptop, and camera.

Another NHK staff member -- whose identity CPJ said it was withholding for safety reasons -- fled Iran in early February after being summoned several times, CPJ said in a statement.

"The arrest of Shinnosuke Kawashima and the intimidation of his colleagues reflects a deliberate effort by Iranian authorities to silence independent reporting," said CPJ Regional Director Sara Qudah.

The reported arrest followed a wave of major protests in Iran in December and January met by a violent government crackdown that rights groups said left thousands of people dead.

This was before the United States and Israeli militaries began "Operation Epic Fury" on February 28 and killed supreme leader Ayatollah Ali Khamenei.

- Risk level -

Japan on Thursday raised its risk level to the second-highest level -- "Avoid all travel" -- for Kuwait, Saudi Arabia (Eastern Province), Bahrain, Qatar, the United Arab Emirates and Oman.

It said it would transport Japanese nationals in Kuwait, Bahrain, Qatar, and the UAE who wish to do so by land to Riyadh and Muscat to take commercial flights.

But with tickets from Riyadh and Oman difficult to get, Japan's government will also arrange charter flights to Tokyo from there.

Tokyo and Tehran have historically had relatively friendly relations, with former premier Shinzo Abe visiting Iran in 2019 and then-president Hassan Rouhani coming to Japan the same year.

As a close US ally, but also a strong supporter of the rules-based international order, the war has put Japan in a difficult position, analysts say.

Prime Minister Sanae Takaichi said Tuesday that she would hold "candid" discussions on the war with US President Donald Trump at their talks in Washington on March 19.

The world's fourth-largest economy is the fifth biggest importer of oil, with around 70 percent coming through the Strait of Hormuz which Iran has effectively closed.