US and Japanese authorities have launched an investigation into reported death threats against Washington's Tokyo ambassador, Caroline Kennedy. Another US diplomat was also threatened, according to a Japanese newspaper.
The US State Department on Wednesday said American and Japanese authorities were working to ensure the safety of embassy staff in Tokyo after a series of threats were made.
The warnings were made by telephone last month in a series of telephone calls to the US embassy, with a threat also made to the US consul general in Okinawa Alfred Magleby.
"We take any threats to US diplomats seriously," State Department spokeswoman Jen Psaki said in a 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," she added, refusing to go into greater detail about the threat.
Japanese broadcaster NTV said the calls threats were made "multiple times" by a man speaking in English.
Kennedy, who is the last surviving child of former US President John F. Kennedy, took up her post in November 2013 and has attracted significant attention because of her family background. She made high-profile visits to the Fukushima nuclear plant and the annual commemoration of the US atomic bombings of Hiroshima and Nagasaki.
Motives remain unclear
Okinawa is home to the majority of US troops stationed in Japan, their presence proving unpopular among many of the southerly island's residents.
The US Embassy and Tokyo Metropolitan Police Department have declined to comment, and there was no indication of the motive given by the State Department.
The warnings came after the US ambassador to South Korea Mark Lippert had his face slashed by an attacker earlier this month at a breakfast event in Seoul. Lippert needed to have 80 stitches and was hospitalized for five days.
Korean prosecutors have recommended that the attacker, described as a nationalist, face an attempted murder charge for the "pre-meditated assault."
News of the Japan threats came to light as US First Lady Michelle Obama arrived in the country as part of a tour to highlight the importance of education for girls.
rc/jil (AFP, AP, dpa)