Police probing Thailand's deadliest bombing issued arrest warrants on Monday for two suspects after a second weekend raid on a suburban apartment block uncovered possible bomb-making materials.

Police were looking for a 26-year-old Thai woman and a foreign man in his 40s after expanding their search to a property in the city's Min Buri district. They found fertiliser, digital watches and an explosives detonator.

That came after a raid on an a decaying apartment building in the nearby Nong Chok district on Saturday, when police arrested a foreigner and seized several kinds of explosives and more than 200 passports.

The Aug. 17 attack on a Bangkok Hindu shrine killed 20 people and injured more than 100. Fourteen foreigners were among those killed in a blast the military government said was aimed at dealing a blow to an already ailing economy for which tourism has become crucial.

A picture of the female suspect showed her wearing a hijab. She rented the room occupied by the foreign man, for whom police issued the second arrest warrant, spokesman Prawut Thavornsiri said. A sketch of the man showed him with cropped hair and a short moustache.

Prime Minister Prayuth Chan-ocha indicated at least one of the two suspects had been caught on closed-circuit cameras on the day of the bombing. However, he did not specify the location covered by the surveillance video.

Police have been criticised for an erratic investigation that had, until this weekend, uncovered few clues about who was behind the blast. No group has claimed responsibility.

The bright pink budget apartment in Min Buri was raided twice at the weekend and media were allowed to observe searches on Sunday while residents, many of them Muslim, were present.

Police have not confirmed the identity or nationality of the 28-year-old man they arrested on Saturday in the busy Nong Chok, area, where many Thai Muslims and foreigners live.

He is charged with possessing illegal explosives. His identity and nationality are unknown.

Army chief General Udomdej Sitabutr said the man was definitely involved in the shrine bombing and authorities were trying to extract information from him, but withholding some to avoid jeopardising the probe.

Speculation on who was involved has focused on southern ethnic Malay rebels, opponents of the government, foreign extremists and sympathisers of Uighur Muslims, among other groups. Thailand drew international outrage last month when it forcibly repatriated more than 100 Uighurs to China.

Many of the minority Uighurs have sought passage to Turkey via Southeast Asia.

"We don't want our home to be the rest stop, or the passing point of unusual migration," Prayuth said. "This is where the network may be hiding. That's why this could also be a reason... a conflict because authorities are becoming more strict."