A woman places a wreath at the Erawan shrine, the site of Monday's deadly blast, in central Bangkok, Thailand, August 20, 2015. International terrorists were not suspected of the bomb attack in Bangkok this week that killed 20 people and China was not the target, Thai authorities said on Thursday, as police said they believed at least 10 plotters were involved.[Photo/Agencies]

BANGKOK - Thai authorities have tripled to $85,000 a reward for information leading to the arrest of the main suspect in the country's worst ever bombing.

Prime Minister Prayuth Chan-ocha said progress in the investigation was being made, but declined to give details.

Police are trying to determine who carried out the Monday evening attack at one of Bangkok's top tourist attractions. Twenty people were killed in the blast at the Erawan shrine, 14 of them foreigners including seven from Chinese mainland and Hong Kong.

"The government insists on speeding up the investigation and bringing the bomber and network to justice," Prayuth said in a weekly address to the nation.

The only solid evidence seems to be grainy security camera footage showing an unidentified young man with shaggy dark hair and dressed in a yellow shirt leaving a backpack at the scene.

Officials have had different theories about the identity of the man, last seen on video footage disappearing into the night on the back of a motorcycle taxi, saying he could be foreign, or a Thai man pretending to be foreign.

The reward for information leading to his arrest was raised to 3 million baht, a police spokesman said.

Initial speculation that the plot could be the work of an international terror network has for now been set aside.

"We still have no information on international terror groups and think there is no link to international terrorism," Somyot Pumpanmuang said after attending a multi-faith prayer ceremony outside a shopping centre near the shrine in central Bangkok.

"What is clear is that it was intended to discredit the government, destroy confidence and make tourists scared and not travel to Thailand," he told reporters.

Asked about his suggestion that at least 10 plotters were probably involved, Somyot said there might only have been two.

CHINESE NOT THE TARGET

The Erawan shrine, dedicated to a Hindu deity, is hugely popular with tourists from China. But the government has said Chinese tourists were not believed to have been specifically targeted. Announcements about the investigation have been broadcast in both Mandarin Chinese and English.

Damage to the tourist industry will not help the military government in its efforts to revive a flagging economy. Police have stepped up patrols in the city.

Two men spotted at the shrine on CCTV and suspected of being the bomber's accomplices have been cleared, but police were now suspicious of a woman dressed in black in the footage, said police spokesman Prawut Thawornsiri.

"Police are asking anyone who is in that CCTV footage to come forward," he said.

($1 = 35.6900 baht)