In testimony during a hearing at the U.S. Securities and Exchange Commission, an SEC official alleged that Deloitte Touche Tohmatsu LLP's failure to turn over audit records has stalled a three-year-old probe into financial fraud at a large solar power company.

"We've had significant delays," said Laura Josephs, an SEC assistant enforcement director.

She added that Deloitte was "continuing to issue audit opinions" both for that company and others, even while it failed to comply with the SEC's request.

In December, the SEC charged the Chinese affiliates of Deloitte, KPMG, PricewaterhouseCoopers , BDO and Ernst & Young with securities violations.

The case, which kicked off Monday in an administrative trial expected to last several weeks, is the SEC's most aggressive effort to date in its regulatory standoff over access to Chinese audit papers.

The United States wants greater oversight after a rash of accounting scandals at U.S.-listed companies based in China have damaged investor confidence.

The SEC is seeking to have the firms' right to practice before the agency suspended - a sanction that could effectively force their U.S.-listed clients to de-list.

CHINA'S SECRECY LAWS INVOKED

The firms argue that they cannot comply with the SEC's request to hand over the audit papers because China's secrecy laws forbid it.

The SEC had previously tried to work things out with Chinese regulators through negotiations, but filed the case against the accounting firms after talks broke down.

Much of the questioning in Monday's hearing focused specifically on Deloitte's audit work for the solar power company, which is referred to in the SEC's complaint as "Client A."

But exhibits displayed in the courtroom on Monday revealed Client A's identity as Canadian Solar, which still trades on the Nasdaq and first disclosed the SEC's investigation in 2010.

A spokesman for the company did not return a call or e-mail seeking comment.

A second mystery client, "Client C," was also identified in court by an Ernst and Young attorney on Monday as the firm Sinotech Energy Ltd., which was sued by the SEC last year and later reached a settlement.

In her testimony, Josephs downplayed the audit firms' claims that Chinese secrecy laws prevent the sharing of documents, saying the agency still managed to get documents from "Client A" upon request.

But Michael Warden, an attorney representing Deloitte, argued that his client and the other audit firms are not violating federal securities laws because their failure to share the documents is not a "willful refusal."

He added that Deloitte willingly turned over 19 boxes worth of audit papers for Canadian Solar to Chinese regulators shortly after the SEC made its request.

So far, Chinese regulators have not shared those documents with the SEC.