TOKYO — The police in Japan are investigating telephoned death threats to the United States ambassador, Caroline Kennedy, the daughter of President John F. Kennedy, and to an American diplomat in Okinawa, Japanese and international news agencies reported on Wednesday.

The reports came as Michelle Obama, the first lady, arrived in Japan on her first visit to the country, and as Ambassador Kennedy, together with former President Bill Clinton, appeared with Prime Minister Shinzo Abe at a Tokyo symposium on the legacy of Ambassador Kennedy’s father, who was assassinated in November 1963 when she was 6 years old.

Concern about the security of American diplomats in Asia was heightened a few weeks ago when the United States ambassador to South Korea, Mark Lippert, was slashed by a knife-wielding assailant in Seoul.

Responding to the reported death threats, the State Department spokeswoman, Jen Psaki, said Wednesday in a statement: “We take any threats to U.S. diplomats seriously. We take every step possible to protect our personnel. 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.”