Japan's tax authorities have begun a review of how slot machine maker and casino developer Universal Entertainment accounted for $40 million in payments made in 2010 to an associate of the Philippines' top gambling regulator at that time, sources told Reuters.

Separately, the payments are also a focus of a previously undisclosed criminal investigation by Hong Kong's Independent Commission Against Corruption, sources said. The ICAC has become involved because the transfers under review passed through bank accounts based in Hong Kong, those people said.

On Wednesday, Universal said in a statement that its U.S.-based subsidiary, Aruze USA, had received a subpoena from a grand jury in the United States that it believed was related to "suspicion" of bribery related to the payments. The company did not say when that subpoena had been received or where that federal grand jury was seated.