The Office of the Attorney-General (OAG) cited a treaty between the two countries as its reason for not revealing information obtained from the US Justice Department.

But Abhisit said his party believed exceptions could be made and the OAG could ask the US to reveal the information for public benefit.

He said the Democrats would support the OAG asking for public disclosure of the information, because the matter was an issue of much public interest and disclosure would create transparency.

He said information from the US showed that a group of people confessed to the offence. And although no politician was named, the document clearly indicated names of companies, managers and agents.

"The point is these people violated Thai law. We must start questioning and prosecute them immediately," Abhisit said.

"And the investigation must extend to cover other people who are believed to be involved in the scandal."

Critics have said the OAG sought to protect the government and was derelict in its duty by failing to reveal the Thai politicians and officials involved in the kickback scandal.

But deputy spokesman Poramate Intarachumnum defended the agency, saying only the Royal Thai Police had the authority to investigate the case. He said the OAG could not reveal documents from the US because a treaty between the two countries banned disclosure for other purposes.

"By accusing us of protecting politicians, it is an insult to permanent officials,'' he said.

Poramate assured the public prosecutors would bring culprits to justice if they were corrupt. He cited the case of former health minister Rakkiat Sukthana, who was sentenced to jail after being convicted of a corruption charge.

"Do not give up hope. If there is wrongdoing, officials will unearth evidence to prosecute them," he said.