Police investigating the deadly Bangkok blast said they were seeking a Thai woman and an unidentified man after bomb-making materials were discovered over the weekend in a second apartment raided by police.

The blast that hit the Erawan shrine in a busy shopping district on August 17 was Thailand's worst single mass-casualty attack, with most of the victims ethnic Chinese tourists from across Asia.

The new lead came after police said they made a breakthrough in their case, detaining an unnamed foreign man on Saturday morning at another flat where detonators, industrial pipes and ball-bearings were found.

In a televised broadcast on Monday national police spokesman Prawut Thavornsiri showed a photograph of the wanted Thai woman, taken from an official identity card, showing her wearing a black hijab.

He named her as 26-year-old Wanna Suansan -- the first time a suspect in the bombing probe has been identified.

A sketch of an unidentified man with a moustache was also broadcast.

Prawut said the two were believed to be renting a second room where more bomb equipment was found, as police search for possible accomplices to the shrine bombing.

"We found fertiliser bags, watches, radio controls -- parts to make bombs and electric charges," Prawut told AFP on Monday, shortly before the national broadcast.

He said the items were found during a raid on an apartment in the northeastern suburb of Minburi over the weekend.

He did not detail when the raid took place but added that the type of fertiliser found was urea based.

Urea nitrate is a compound commonly used in homemade bombs.


'Hunt narrows'

Mystery has surrounded the unprecedented attack on Thai soil, for which no group has claimed responsibility.

Thai authorities have played down any suggestion the attack was launched by international terrorists or specifically targeted Chinese tourists.

Potential perpetrators named by the police and experts before the man was arrested have included international jihadists, members of Thailand's southern Malay-Muslim insurgency, militants on both sides of the country's festering political divide or someone with a personal grudge.

Media accompanied police during a search of multiple flats in Minburi on Sunday but no items were shown to the press or announcement made of any evidence discovery.

The area is near to Nong Chok, another suburb where the unidentified foreigner was arrested on Saturday.

Both districts are mixed suburbs with significant Muslim populations.

Police believe the suspect in custody, who pictures showed was thin with heavy stubble, was part of a crime group who helped illegal migrants obtain counterfeit documents -- and that the bomb attack on the shrine was retaliation for a recent crackdown on their lucrative trade.

Experts say that motive is unlikely because it would bring further scrutiny to such an organisation's operations.

Police say they are working with "several embassies" to try to ascertain the identity of the man, who is being held in military custody.

Officials initially said he was not cooperating with his interrogators as they brought in multiple translators -- including an English speaker -- to help with interviews.

But on Monday police struck a more upbeat note.

Police chief Somyot Poompanmoung told reporters that the suspect's interrogation had yielded "very useful" information.

Analysts have speculated the blast, which has not been claimed by any group, could have been motivated by revenge over Thailand's recent deportation of scores of ethnic Uighurs to China.

In a televised briefing on Monday, a spokesman for Thailand's ruling junta said authorities had "not ruled out any scenarios."