By Rory Carroll

SAN FRANCISCO, Dec 29 (Reuters) - Ambac Assurance warned Puerto Rico's governor on Tuesday over what it said what his illegal plan to take rum tax revenue pledged to the Puerto Rico Infrastructure Authority (PRIFA) and use it to meet other financial obligations ahead of a key debt payment on Friday.

Ambac Assurance and Financial Guaranty Insurance Company (FGIC) and its parent company, Ambac Financial Group, insure more than $863 million in aggregate outstanding principal amount of bonds issued by PRIFA.

Those bonds are secured by the taxes on rum and other articles produced in Puerto Rico and sold in the United States.

Governor Alejandro Garcia Padilla on Dec. 1 granted the U.S. territory the power to take revenues from public agencies like PRIFA via "clawbacks."

In a letter to Garcia Padilla and members of PRIFA, the companies complained that as much as $94 million in rum taxes securing the PRFIA bonds were diverted even before Padilla announced the policy change, saying no money had come into a bondholder trust account known as a "sinking fund" since the beginning of September.

"Ambac and FGIC demand that the Rum Taxes received in fiscal year 2016, up to the statutory $117 million limit, be immediately transferred to the sinking fund," the letter said.

A Padilla spokesperson did not respond to a request for comment.

Earlier this month, Melba Acosta, president of the island's Government Development Bank (GDB), was quoted in local media saying the island was expected to default on a Jan. 1 payment on its PRIFA bonds.

Puerto Rico first defaulted on its debt in August and Garcia Padilla has warned that more defaults are coming. The island has an upcoming debt payment of about $1 billion due on Friday.

(Reporting by Rory Carroll; Editing by Peter Cooney)