NEW YORK : US authorities Wednesday hit Credit Suisse with a record fine of 536 million dollars for "egregious" violations of US sanctions on Iran and other countries over a 20-year period.

The settlement with the Swiss banking giant represents the largest penalty settlement in the history of the office monitoring violations of US sanctions, officials said.

The US Treasury and other state and local agencies concluded that the Swiss bank helped facilitate payments to Iran and other countries subject to US sanctions by disguising the transactions.

"This case arises out of Credit Suisse's processing of thousands of transactions over a 20-year period that concealed the involvement of sanctioned parties and the routing of wire transfers and securities transactions to and through the United States," a Treasury statement said.

"The great majority of the transactions involved Iran, although there were also transactions that appear to have violated US sanctions on Sudan, Libya, Burma, Cuba, and the former Liberian regime of Charles Taylor."

Credit Suisse, which indicated on Tuesday that it was preparing the settlement, agreed to pay 268 million dollars each to the United States and to New York state for a total of 536 million dollars, officials said.

"The efficacy of our sanctions depends upon vigorous compliance and enforcement," said Adam Szubin, director of the Treasury Department's Office of Foreign Assets Control.

"Credit Suisse routed transactions through the United States surreptitiously, while knowing that those payments would be blocked or rejected if their true nature had been clear. These practices were not reasonable or authorized under US law."

Officials said Credit Suisse used elaborate procedures for altering payments to hide their true destination, including stripping out the names of sanctioned parties from payment instructions.

Authorities said Credit Suisse's London affiliate processed trades through the bank's US branch on behalf of a then-designated Libyan state-owned investment company and a Sudanese bank.

Authorities also contended that Credit Suisse sought to take over Iran's US dollar clearing business when another international bank had stopped its operations in 2003.

"As a result, the number of apparently illicit payments involving Iran that passed through the United States dramatically increased from 2002 to 2005," the Treasury statement said.

"This matter represents an egregious case under OFAC's enforcement guidelines due to a number of aggravating factors, including the substantial economic benefit to sanctioned parties, the scope and severity of the apparent violations, and the awareness of the conduct within the bank."

But officials said Credit Suisse cooperated with US investigators and could have faced an even stiffer penalty if it had refused.

On Tuesday, Credit Suisse said it had already made policy changes to come into compliance with US sanctions including closing its representative office in Tehran.

"Credit Suisse has previously disclosed the investigation by US authorities and that it was conducting an internal review into certain US dollar payments involving countries, persons or entities that may be subject to US economic sanctions," the firm said.

Credit Suisse added that it had exited "the business in question" in December 2005 and launched "an extensive independent investigation into the Zurich-based payment activity and other practices.

The settlement was announced by the US Treasury, Department of Justice and Federal Reserve as well as the New York County District Attorney's office.

The Swiss Financial Market Supervisory Authority, the home country supervisor of Credit Suisse, agreed to assist US officials in implementation and supervision of the agreement.