U.S. Ambassador to Japan Caroline Kennedy stresses the importance of the partnership between the U.S. and Japan in the success of promoting equal opportunities for women worldwide during a news conference Thursday, March 19, 2015, at the Iikura Guest House in Tokyo. At left is Japanese first lady Akie Abe.

TOKYO (Tribune Content Agency) — Japanese police arrested a resident of Okinawa over death threats against U.S. Ambassador Caroline Kennedy, as an almost two-decade row over the expansion of an American military base rumbles on.

The 52-year-old man used a public phone in Naha, the capital of the southern Japanese island, to make anonymous calls in English and threatened to blow up the embassy building in Tokyo, the Jiji news agency reported. The U.S. State Department welcomed the arrest as a “positive step.”

The threat comes after the U.S. envoy to South Korea, Mark Lippert, was slashed in the face before a speech in Seoul earlier this month, highlighting the risks faced by diplomats globally. It was accompanied by a threat to blow up the U.S. Marine base in Okinawa, where expansion plans are being protested by local residents who say they bear an unfair share of the burden of the American military presence in Japan.

“We take any threats to U.S. diplomats and U.S. diplomatic facilities very seriously,” Jen Psaki, a State Department spokeswoman, told reporters in Washington. “We’ve been working with the Japanese government on these threats since prior to Ambassador Lippert’s attack.”

Protests have been flaring at the Okinawan marine base since the resumption of work to reclaim land to build new runways at the facility, meant to replace Futenma Air Station in a crowded city on the island. Okinawa hosts about three-quarters of the U.S. bases in Japan on only 0.6 percent of the country’s land mass.

Kennedy met with Michelle Obama on March 19 as part of the U.S. first lady’s trip to Japan and Cambodia to draw attention to the Let Girls Learn education initiative. The ambassador is scheduled to attend a ceremony Saturday to mark the 70th anniversary of the battle of Iwo Jima.

Kennedy, 57, became ambassador to Japan in November 2013. She is the only surviving child of U.S. President John F. Kennedy, who was assassinated in 1963.

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