Tokyo police are investigating death threats against U.S. Ambassador to Japan Caroline Kennedy and another American diplomat, according to media reports.

The U.S. Embassy in Tokyo received the threats by telephone last month, according to the Yomiuri Shimbun newspaper. There were several phone calls made by an English-speaking man, the Japanese newspaper said Tuesday, citing a Tokyo police official.

The threats also targeted the U.S. consul general in Okinawa, Alfred Magleby, according to Yomiuri and U.S. media reports on Wednesday.

Reuters could not immediately confirm the reports.

The threat reports come two weeks after the U.S. ambassador to South Korea, Mark Lippert, was injured after being attacked by a knife-wielding assailant during an event in Seoul.

That attack has prompted debate about threats against American diplomats abroad and security precautions taken to protect U.S. ambassadors in particular. The State Department has said security for Lippert had been adequate.

The reports emerged as first lady Michelle Obama was set to arrive in Japan for a three-day visit.

Representatives for the State Department could not be immediately reached for comment.

U.S. Ambassador to Japan Kennedy looks on before a welcome dinner hosted by Japanese Prime Minister Abe in Tokyo