The bomb attack that killed 20 people at Bangkok's Erawan shrine on Monday was carried out by a "network", Thailand's chief of police has said. Police have released a sketch of the main suspect, a man in a yellow T-shirt who was filmed by security cameras leaving a backpack at the shrine.

Prime Minister Prayuth Chan-ocha has described the incident as the worst-ever attack on Thailand.

Police later said two other people seen on CCTV were being treated as suspects.

Police chief Somyot Poompanmoung told our source that the shrine attack was the work of more than one person.

"He didn't do it alone for sure," he said, referring to the main suspect. "It's a network," he added, without giving further information. But he said he was certain that Thai citizens were involved in the bombing.

Although the main suspect in the footage has not been identified, an arrest warrant for him has been issued by Bangkok's Southern Criminal Court. It accuses an "unnamed foreigner" of conspiracy to commit "premeditated murder" and weapons offences.

Police spokesman Prawut Thawornsiri said in a TV interview that the man could be of "mixed origin".

A reward of one million baht ($28,000; £17,950) has been offered for information leading to his arrest.

Mr Prawut also said that two other men seen in the grainy CCTV footage were being sought. "The person in red and the person in white are also suspects," he said.

The BBC's Steve Evans in Bangkok says the new information implies this may have been an organised international operation, perhaps with a religious motive.

The prime minister urged the main suspect to surrender to the police because he might otherwise "get killed to stop him from talking".

Mr Prayuth told reporters the man must have been hired to plant the bomb.

However, our correspondent says there are indications the main suspect was driven to the airport after the attack and may well have flown.

In a separate attack on Tuesday, an explosive device was thrown at a pier in Bangkok. No-one was hurt, but the authorities have not ruled out a link between the incidents.

Maj Gen Weerachoon Sukhontapatipak, a spokesman in the military government, earlier told the BBC that the authorities were "quite close" to identifying the suspect caught on CCTV at the shrine.

The man is seen carefully removing his backpack inside the shrine - at the spot the bomb went off - then getting up without it and immediately leaving.

Gen Sukhontapatipak said no motive was being ruled out and the character of the bombing was "quite different" from previous attacks by Islamist separatists in southern Thailand.

Speaking in a television address on Tuesday evening, Mr Prayuth said the attack showed that Thailand "still has a person or a group of people with hostility to the nation operating actively".

"They may be doing it for a political motive or to undermine the economy or tourism or for other reasons," he said, according to the Bangkok Post.

He also warned against speculation on possible perpetrators, saying it could cause panic or mislead the investigation.