A second foreign man has been arrested at a checkpoint on the Cambodian border over last month’s deadly shrine bombing, Thailand’s prime minister says.

It was the second confirmed arrest in connection with the August 17 blast, which tore through a religious shrine in central Bangkok and killed 20 people, mostly ethnic Chinese tourists.

Prayut Chan-ocha, who also heads the nation’s junta, was asked by reporters on Tuesday to comment on rumours a man had been detained.

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“It’s true. He has been arrested at Sa Kaeo checkpoint,” Prayut said, referring to the Thai side of a border crossing with Cambodia.

Asked whether he is thought to be the person who planted the bomb at the Erawan shrine in Bangkok’s busy Chidlom shopping district, he replied: “We are interrogating. He is a main suspect and a foreigner.”

The motive for the blast, Thailand’s worst single mass-casualty attack, remains shrouded in mystery.

The hunt for the perpetrators of the bomb blast has been characterised by confusing and at times contradictory statements from police and junta officials.

In recent days police appeared to have narrowed their focus.

Arrest warrants have been issued for several people including three unnamed males.

Thailand’s national police spokesman Prawut Thavornsiri shows a picture of Wanna Suansan, wanted for questioning. Photo: AAP

The only named warrant is for a 26-year-old Thai Muslim woman called Wanna Suansan whom police say rented a flat where bomb-making equipment was found.

The first foreign suspect was detained on Saturday at another nearby flat, allegedly in possession of bomb making paraphernalia and dozens of fake Turkish passports.

He is in military custody but has not been publicly identified.

Thai authorities confirmed on Tuesday that Wanna is currently overseas but refused to say in which country.

Late Monday, AFP tracked down her number and a woman answering that name took the phone call, saying she was living in the Turkish city of Kayseri with her husband whose nationality she did not state.

The number was for a Turkish mobile phone.

National police spokesman Prawut Thavornsiri told reporters Tuesday that officers had searched her parents’ home in the southern province of Phang Nga.

“Her parents said she was not there, she was abroad. We are co-ordinating for her to come talk to police,” he said, without revealing her presumed whereabouts.

In her phone interview Wanna denied involvement in the blast, saying she had not visited the flat where the bomb-making equipment was found for around a year.

A junta spokeswoman said in a broadcast the ongoing interrogation of the foreigner detained on Saturday had led to “the apprehension of additional suspects”.

The broadcast was made before the announcement by Prayut.