TOKYO >> Japanese police are investigating phone calls threatening to kill U.S. Ambassador Caroline Kennedy and another American envoy, authorities said Wednesday.

Tokyo police are looking into calls to the U.S. Embassy threatening to kill Kennedy and similar ones targeting Alfred Magleby, the U.S. consul general based on the southern island of Okinawa, an Okinawa police official said on condition of anonymity because he was not authorized to comment on an investigation by Tokyo authorities.

Tokyo police declined to comment. The embassy also did not comment, citing policy regarding the ambassador’s security.

State Department spokeswoman Pen Psaki said the U.S. government takes threats to American diplomats seriously.

"We are working with the Japanese government to ensure the necessary measures are in place. We will not comment on the specific details of any threats or the steps we take to address them," she said.

Okinawa is home to about half of the 50,000 American troops based in Japan.

Japanese media reports said that the death threats came last month from a caller speaking in English, and that police were looking into the case on suspicion of blackmailing. No other details, including motives, were known.

Earlier this month, the U.S. ambassador to South Korea, Mark Lippert, was knifed by an anti-U.S. activist in Seoul and had to be hospitalized for several days.

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Associated Press writer Matthew Lee in Washington contributed to this report.