LONDON (Reuters) - U.S. authorities are investigating whether Standard Chartered STAN.L breached Iran-related compliance rules as recently as 2013, a year after it settled with them over earlier allegations of breaches and pledged to improve internal controls.

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Standard Chartered’s chief executive Bill Winters said in a memo to senior staff addressing recent media reports, that “the vast majority of the payments under investigation pre-date 2012, and none occurred after 2014”, in the first public acknowledgement by the bank of the later timeframe.

Media reports last week said that London-based Standard Chartered faced a possible $1.5 billion fine for Iran-related sanctions violations, in addition to the $667 million it paid in 2012 to settle alleged breaches between 2001 and 2007.

The investigation now underway focuses on breaches after 2007, with special emphasis on whether the U.S. authorities were fully informed about the bank’s dealings with Iranian clients at the time of the 2012 settlement.

A spokesman for Standard Chartered confirmed the contents of Tuesday’s memo, which was sent to hundreds of Standard Chartered’s senior managers, but declined to comment further.

The memo from Winters, which was seen by Reuters, also addresses media coverage of Standard Chartered’s attempts to improve its financial crime controls.

“We have been cooperating with an investigation related to historical conduct and control issues. Resolving this legacy matter remains a key priority,” Winters said.

“It has been a long process, which cannot be helped, but we are closely engaged in constructive, ongoing discussions with the authorities to reach an acceptable resolution,” he added

The media reports said the fine of around $1.5 billion was a preliminary assessment based on some of the communications between the bank and regulators.

Standard Chartered has declined to comment on the size of the possible fine but said last week it was “engaged in ongoing discussions with U.S. authorities”.

The bank has been subject to a deferred prosecution agreement (DPA) with the United States since 2012.

The DPA, which carries the threat of a bigger penalty if Standard Chartered breaks the terms of the settlement, is due to expire in December after it was extended in 2014 and in 2018.

Standard Chartered warned shareholders in its most recent annual report that resolving the U.S. investigation could mean substantial monetary penalties.