The U.S. State Department is working with Japanese authorities after death threats against Ambassador Caroline Kennedy were phoned into the U.S. Embassy in Tokyo.
"We take any threats to U.S. diplomats seriously," Jen Psaki, State Department spokeswoman, said in an emailed statement. "We take every step possible to protect our personnel. We are working with the Japanese government to ensure the necessary measures are in place."
U.S. officials wouldn't comment on the specific details of any threats or steps they are taking to address them.
According to media reports, Tokyo police are investigating calls made in February threatening to kill Kennedy and similar ones targeting Alfred Magleby, the U.S. consul general based in Okinawa.
The calls were made by someone with a male voice who was speaking English and are being investigated as a suspected case of blackmail, the reports said.
The threats come on the heels of the March 5 attack on the U.S. ambassador to South Korea, Mark Lippert, who required 80 stitches and was hospitalized for several days after being knifed by an anti-U.S. activist.
The Metropolitan Police Department in Tokyo declined to comment.
Kennedy, 57, became ambassador to Japan in November 2013. She is the only surviving child of the late U.S. President John F. Kennedy, who was assassinated in 1963.
First lady Michelle Obama is now in Japan on a trip to promote the Let Girls Learn education initiative.