Japanese police are investigating telephoned death threats against U.S. Ambassador Caroline Kennedy. 

Authorities told the Associated Press Wednesday that Tokyo authorities are investigating the calls made to the U.S. Embassy in the Japanese capital. They said the embassy had received similar calls targeting Alfred Magleby, the consul general based on the southern island of Okinawa, home to about half of the 50,000 American troops based in Japan.

Tokyo police declined to comment on the threats. A State Department official, who spoke on condition of anonymity because he was not authorized to discuss security precautions, told the AP that the Tokyo police department provides an armed security detail for Kennedy.

Japanese media reported that the calls to the embassy were made last month by a man who spoke English. The reports suggested that the threats may have been tied to a scheme to blackmail Kennedy or the embassy. 

"We take any threats to U.S. diplomats seriously," said State Department Deputy Spokesperson Marie Harf. "We take every step possible to protect our personnel. We are working with the Japanese government to ensure the necessary measures are in place."

The threats against Kennedy have been made public 13 days after Mark Lippert, the U.S. envoy to South Korea, was attacked and injured by a knife-wielding assailant. South Korean police said the attacker had targeted Lippert to protest joint military drills conducted by the U.S. and South Korea. 

The 57-year-old daughter of former President John F. Kennedy assumed the Tokyo post in 2013 after being nominated by President Barack Obama. 

Kennedy has drawn considerable attention since arriving in Japan. She has made well-publicized visits to the Fukushima nuclear plant devastated by a 2011 earthquake and tsunami, and to the annual commemorations of the U.S. atomic bombings of the cities of Hiroshima and Nagasaki.

Kennedy visited Okinawa in February last year in an attempt to win support for the base relocation plan, and pledged that Washington would do its best to reduce the burden of its heavy troop presence.

Okinawa residents have frequently complained about crime, noise and other issues related to the U.S. bases. The U.S. government wants to relocate one base, the Marine Corps Futenma air station, to another area of Okinawa, but many people want it moved completely off the island.

The Associated Press contributed to this report.