News website The Intercept reported that the US National Security Agency and Britain's Government Communications Headquarters hacked into Gemalto's systems to steal encryption keys that could unlock security settings on billions of mobile phones.
The website cited documents provided by American whistle blower Edward Snowden as the basis for its report.
The Japanese company began using SIM cards produced by Gemalto in 2001, DoCoMo spokesman Takashi Itou said in an e-mail in response to questions from Reuters. He declined to say what share of DoCoMo mobile phones use SIM cards supplied by the Franco-Dutch supplier.
"We will consider any necessary steps based on the results of our investigation," Itou said.
Gemalto said it was conducting its own investigation of the reported hack by American and British spies.
The SIM, short for subscriber identity module, is a chip that acts as an electronic ID for the mobile phone and makes it identifiable to the mobile network operator.
Gemalto competes with several European and Chinese SIM card suppliers. It produces around two billion SIM cards a year and counts Verizon, AT&T Inc and Vodafone among its customers. — Reuters