An image released by the Royal Thai Police on August 19, 2015, shows a sketch of the main suspect in Monday's deadly blast, in central Bangkok, Thailand, Aug 19, 2015.[Photo/Agencies]

BANGKOK - Thai police said on Wednesday that a suspect captured by CCTV cameras minutes before a bomb exploded at Bangkok's Erawan shrine was a foreigner, and his appearance suggested he might be from Europe or the Middle East.

Police spokesman Prawut Thawornsiri also said investigators were now convinced two other men seen on the grainy video footage were accomplices.

At least 20 people were killed in Monday's blast in the heart of the Thai capital. The government says the unprecedented attack on the city was designed to wreck the country's economy, which depends heavily on tourism.

No one has claimed responsibility for the evening rush-hour bombing, in which 11 foreigners from several Asian countries died and more than 120 were wounded.

Prawut said in a televised interview that an arrest warrant had been issued for a "foreign man", a sketch of whom showed a fair-skinned youth with thick, medium-length black hair, a wispy beard and black glasses.

"He had white skin and must have been a European or have mixed blood, perhaps with Middle Eastern blood," Prawut said, without giving a reason for his assumptions other than the colour of the man's skin.

The sketch was based on footage that showed a man dressed in a yellow T-shirt dumping a backpack inside the shrine compound and walking away through a crowd of tourists about 20 minutes before the explosion.

Prawut earlier tweeted that police were offering a 1 million baht ($28,100) reward for information leading to the arrest of the suspect.

He said two other men, one dressed in red and another in white, were seen milling around the first suspect, apparently shielding him from the view of the crowd as he placed the rucksack in front of a railing.

Earlier, police had said they were sure some Thais were involved in the attack.

The shrine, a blood-spattered scene of charred motorbikes and debris after the blast, was reopened on Wednesday.