James Giddens, trustee for the U.S. broker-dealer unit, said in an emailed statement the new estimate stemmed from claims from customers who traded on both U.S. and foreign exchanges, with about $700 million still in dispute with the British administrators of MF Global UK Ltd.

It was not immediately clear how this affected Giddens' previous estimate of around $1.2 billion in missing U.S. customer funds. The trustee's spokesman was not available for comment.

The statement said the trustee has so far returned about $3.9 billion in commodity customer funds, and said about 91 percent of outstanding claims were for less than $100,000.

"The Trustee is eager to make additional distributions to former MF Global Inc customers as soon as possible," the statement said.

"However, the Trustee is required by law to hold an appropriate reserve of funds until disputed claims are resolved either through negotiation or by the Court. At this time, the Trustee anticipates significant disputed claims against the MF Global Inc. estate by MF Global Holdings Ltd., MF Global UK Limited, and other entities."