Still image from closed-circuit television (CCTV) footage released by Thai police shows what they believe to be two new suspects - dressed in red (C) and dressed in white (3rd R) - standing in front of previously identified supect in yellow t-shirt removing his backpack prior to a bomb blast in Bangkok, made available August 19, 2015.[Photo/Agencies]

However, Prime Minister Prayuth Chan-ocha sought to cool speculation of a revenge attack by Uighurs.

"I have always said that what the government did was within the boundaries of the law and by international agreement," he told reporters. "If we did not send them they would have been a burden to Thailand. I don't want this issue raised."

Police chief Somyot Pumpanmuang said no direct connection had been established between Monday's deadly blast and a small explosive thrown from a bridge towards a river pier in Bangkok on Tuesday. No one was hurt by that second bomb. A government spokesman had initially said there were "patterns" linking the two bombs which both used TNT.

The blast comes at a sensitive time for Thailand, which has been riven for a decade by a sometimes-violent struggle for power between political factions in Bangkok.

A parliament hand-picked by a junta that seized power in a 2014 coup is due to vote on a draft constitution next month. Critics say the draft is undemocratic and intended to help the army secure power and curb the influence of elected politicians.