BANGKOK (Reuters) - The Thai authorities have tripled to 3 million baht (S$118,230)a reward for information leading to the arrest of the main suspect in the country's worst-ever bombing.

The government said progress in the investigation was being made, but declined to give details or offer evidence they were closer to determining who carried out the Monday evening attack at one of Bangkok's top tourist attractions.

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

Twenty people were killed in the blast at the Erawan shrine, 14 of them foreigners, including seven from mainland China and Hong Kong.

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

Officials have speculated that the man, last seen on video footage disappearing into the night on the back of a motorcycle taxi, could be foreign, or a Thai man pretending to be foreign.

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

And on Friday, police chief Somyot Pumpanmuang appeared to backtrack on his suggestion the previous day that the bomber was probably part of a network of at least 10 people who spent a month planning the attack.

"We still have no information on international terror groups and think that there is no link to international terrorism,"Mr Somyot told reporters 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.

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 with information and tell us what they observed that night," he said.

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

The Erawan shrine, dedicated to a Hindu deity, is hugely popular with tourists from China.

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.