The trial of two Chinese Uighurs accused of killing 20 people when they allegedly bombed a Bangkok shrine was postponed Tuesday because the men still do not have a translator.

The delay is the latest snag in a cryptic case that has so far shed little light on the horrific attack in Thailand's capital last year that also left 100 people wounded.

More than a dozen ethnic Chinese were among the dead when explosives -- apparently left in a backpack -- detonated in a Hindu shrine popular with tourists, in August 2015.

The blast came weeks after Thailand's junta forcibly repatriated 109 Uighurs to China, where rights activists says the Turkic-speaking Muslim minority face cultural and religious repression.

Thailand had long been a transit hub for Uighurs fleeing China, most heading for Turkey.

But since the 2014 coup, which sparked criticism from western allies, Thailand's military leaders have grown closer to Beijing.

Junta authorities have been criticised for a murky investigation that appeared to wind down shortly after the arrest of the two men, leaving more than a dozen key suspects at large.

Yusufu Mieraili and Bilal Mohammed have denied all charges, and have accused their jailers of beating them and denying them halal food in the military prison where they have been held for the past year.

Authorities deny the two men have been mistreated.

Their case was further complicated when their translator, an Uzbek national, fled after he was hit with drug possession charges in June.

Sirojiddin Bakhodirov accused police of planting drugs on him as punishment for helping Thailand's Uighur community -- a charge officers denied.

"He did not come to the court today so the trial needed to be postponed so that we can find a new translator," defence lawyer Schoochart Kanpai told reporters outside the courtroom.

The judge agreed to table the proceedings until the next hearing date on 15 September, according to an AFP reporter inside the courtroom.

The postponement "reflects poorly on the judiciary's preparations to try these suspects", said Brad Adams, Asia Director at Human Rights Watch.

"Everyone knew that there were no other translators immediately available when the previous person was arrested on drug charges. This raises concerns for the suspects' right to justice, fairly and speedily served," he told AFP.

Prosecutors accuse Mohammed of placing the bomb inside a backpack at the shrine and say Mieraili was involved in transporting the device.

They say the bombing was carried out by a people-smuggling gang angered by a police crackdown.