Washington's withdrawal is set to take effect on December 21, 2018, when it will establish an "observer mission" to replace its representation at the agency.
The State Department briefly outlined its reasoning in a press release: "This decision was not taken lightly, and reflects US concerns with mounting arrears at UNESCO, the need for fundamental reform in the organization, and continuing anti-Israel bias at UNESCO".
Since 2011, United States arrears to the organization have reached about $600 million, Ms. Bokova said, but she had told members of Congress repeatedly that immediate payment was not an issue, only American political re-engagement in the organization, which she believes serves many American interests overseas.
"The universal nature of UNESCO's activities is important in protecting peace and ensuring worldwide security in the face of violence and hatred", Bokova said in her statement.
She paid tribute to what she said had been a meaningful relationship between UNESCO and the U.S., saying: "since 2011, we have deepened the partnership between the United States and UNESCO, which has never been so meaningful".
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While the Trump administration had been preparing for a likely withdrawal from UNESCO for months, the timing of the State Department's statement Thursday was unexpected.
The U.S. joined UNESCO at its creation in 1945.
While the United States withdrew from the group, the Trump administration said it wanted to continue providing American perspective and expertise to Unesco, but as a nonmember observer. And in 2015, Unesco adopted a resolution that criticized Israel for mishandling heritage sites in Jerusalem and condemned "Israeli aggressions and illegal measures against freedom of worship".
Many saw the vote to include Palestine as evidence of long-running, ingrained anti-Israel bias within the United Nations, where Israel and its allies are far outnumbered by Arab countries and their supporters. "The organisation's absurd and shameful resolutions against Israel have consequences".
Worldwide, UNESCO also promotes education, gender equity initiatives, access to culture and science, and the pursuit of liberal democratic ideals like freedom of expression. The city, one of the world's oldest, sits in the Israeli-occupied West Bank.