The man Thai police believe masterminded last month's deadly Bangkok bombing fled to Bangladesh the day before the attack and travelled on a Chinese passport, officials said on Wednesday.

The man emerged as a key suspect in the Aug. 17 bombing following interrogation this week of one of two foreigners being held, who police say admitted to giving a backpack with explosives to a man they are certain was the bomber.

The suspect told police a man called "Izan" played a lead role and assigned responsibilities to others plotters during a Bangkok meeting.

"This man called Izan - and I don't know if this is his real name - is a very important person in this network," deputy police chief Chakthip Chaijinda told Reuters.

"I don't know what his nationality is ... Let's just say Izan is one of the foremost wanted individuals."

No group has claimed responsibility for the blast at the Erawan Shrine, a tourist attraction close to hotels and shopping malls. Twenty people were killed, including 14 foreign tourists.

Chakthip said police would be coordinating with Bangladeshi counterparts where the suspected leader had travelled on Aug. 16. An immigration official said he had used a Chinese passport, but it was not known if it was authentic.

A K M Shahidul Hoque, inspector general of Bangladesh police, told Reuters he was unaware of the new development.

Thai police are no closer to establishing a motive for the bombing after weeks of conflicting theories and false leads.

Arrest warrants have been issued for 11 suspects, but only two have been detained following raids that uncovered bomb-making materials.

National police chief Somyot Pumpanmuang reiterated the attack was "not an act of international terrorism".

The most progress has been a suspect admitting to delivering a backpack to a man caught on security footage leaving it at the shrine before the blast.

Police spokesman Prawut Thawornsiri said his testimony revealed the mastermind had given directions to others using the instant messaging platform Whatsapp, but suspects' phones had left no trail.

Prawut said the perpetrators may have been human traffickers and the Chinese passport used by one suspect, which gave the western region of Xinjiang as birthplace, was likely to be genuine.

The use of Chinese passports has raised speculation the attackers could be sympathisers of Uighur Muslims from Xinjiang, who complain of persecution.

Thailand in July forcibly returned 109 Uighurs to China, which denies persecuting the community.

Seven of those killed in the blast were from mainland China and Hong Kong.