Wal-Mart Stores Inc has built a network of 78 subsidiaries and branches in 15 offshore tax havens to minimize taxes on its operations outside the United States, a report by tax reform advocacy group Americans for Tax Fairness said.

Walmart owns at least $76 billion in assets through shell companies domiciled in Luxembourg and the Netherlands, the report said.

The world's largest retailer does not list these subsidiaries in its annual filings with the U.S. Securities and Exchange Commission.

Subsidiaries in tax havens own at least 25 of Wal-Mart's 27 foreign operations, in countries such as the United Kingdom, China and Japan, the report said.

Wal-Mart has 22 shell companies in Luxembourg, where it does not have a single store, and paid less than 1 percent in tax in the country on $1.3 billion in profits from 2010 to 2013, the report said.

Wal-Mart was not immediately available for comment.