BANGKOK: This is the face of Bangkok's mass killer.

Thailand's prime minister urged the man who planted Monday's bomb at a shrine in central Bangkok to surrender or risk being killed by his accomplices to guarantee his silence.

"I would like to tell the bomber that if he wants to be safe, then he should turn himself in to officials and they will find a way to protect him," Prayuth Chan-ocha told journalists.

Mr Payuth, late on Wednesday, played down suspicion that Uighur Muslims were behind the attack in retaliation of Thailand deporting more than 100 Uighurs from Thailand to China in July.

Earlier police had made Uighur theory a key focus of their investigations, according to Thai meda.

The prime minister said the suspect may have been paid to plant the bomb.

Police released this sketch of the fair-skinned, skinny man wearing a yellow T-shirt who was seen in CCTV footage leaving a backpack and walking way from the open-air Erawan shrine before a huge blast that killed about 20 people and injured 120 others, an attack that has rattled nerves in the Thai capital.

Officers sealed off a section of Bangkok on Wednesday, causing traffic chaos, when a suspicious package was found that later turned out to be an empty box.

Police have offered a US$28,000 reward for information that leads to the arrest of the suspect who had thick, medium length hair, a wispy beard and black glasses.

They said he may have been wearing a wig. It was unclear whether he was a Thai or foreigner.

A massive hunt is underway for the man across the country and border posts are on high alert.

Police said that two other men seen in CCTV footage near the scene of the blast are also suspects.

They have not released descriptions of the suspects but said one was wearing white and the other red.

National police chief Somyot Pumpanmuang said it was not clear how many people were involved in the attack or if they were still in the country.

"I don't suspect one person, I suspect many people . . . it's a network," he said.

"I am confident that there are Thais involved but I am not saying it is just Thais or that there are foreigners," he said.

Mr Somyok declined to elaborate.

No one has claimed responsibility for the bombing or a second failed bombing attempt at a river bridge in Bangkok on Tuesday afternoon.

Thai media has quoted police sources saying a key focus of the investigation is a revenge attack by Uighur Muslims.

Thailand infuriated Uighur groups and sparked international condemnation in July when the country deported 109 Uighurs to China.

Protests forced Thailand to close its embassy and consulate in Turkey after photographs showed Uighurs being led hand-cuffed and hooded on to two planes by security guards.

Thailand officials cited intelligence from the country's Special Branch that there would be an attack on Chinese tourists in Thailand after August 11.

Chinese were among tourists at Erawan shrine.

But Thai authorities said they are not ruling out any other groups, including elements opposed to Thailand's military government.

Malay-Muslims insurgents have been waging a bloody war against Thai security forces in the country's southernmost provinces near Malaysia that has left more than 6000 people dead and 11,000 injured.

But the insurgents have rarely in the past ventured out of the four provinces and have shunned attempts to become aligned with international terror groups like al-Qaeda.

Analysts now worry that international terrorists may have helped elements of the insurgents build the bomb that appears to have been made with TNT and was packed with ball bearings.

This would mark a dangerous escalation of the conflict.

Thailand's government has said the bombing was an attempt to sabotage Thailand's economy and tourist industry.