A former senior colleague of billionaire Warren Buffett has been forced to step down from one of the largest funds in the US following a probe by the Securities and Exchange Commission into transactions at her previous employer, Berkshire Hathaway.

Elizabeth Monrad resigned as chief financial officer of the influential New York-based Teachers Insurance and Annuity Association-College Retirement Equities Fund after the SEC started probing transactions between General Re, the reinsurance unit of Buffet's Berkshire Hathaway, and American International Group.