While the U.S. stopped funding Unesco after it voted to include Palestine as a member in 2011, the state department has maintained a Unesco office at its Paris headquarters and sought to weigh in on policy behind the scenes.
UNESCO, which is best known for producing the list of World Heritage sites that includes the Grand Canyon and other USA attractions, has been the scene of diplomatic flare-ups in recent years after Arab countries succeeded in passing a number of resolutions critical of Israel.
"This decision was not taken lightly, and reflects United States concerns with mounting arrears at UNESCO, the need for fundamental reform in the organisation, and continuing anti-Israel bias at UNESCO", she said in a statement.
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.
"This truth has become even more evident today, at a time when the growth of extremism and terrorism calls for new long-term solutions for peace and freedom, in the context of combating racism and anti-Semitism, in combating ignorance and discrimination", she said.More news: Creed 2 filming date & director announced
Its head, Irina Bokova, earlier called the U.S. withdrawal a matter of "profound regret".
"This decision was not taken lightly", according to a State Department statement on Thursday.
Ms Bokova's two terms as director have been deeply scarred by the 2011 Unesco vote to include Palestine as a member, funding troubles and repeated resolutions seen as anti-Israel.
She said the USA pulling out is a loss for the United Nations and for global alliances.
In a statement released via her official Twitter handle, Bokova called the withdrawal "a loss to UNESCO".More news: Now Google Finds Trace Of Russian Bought Ads On YouTube, Gmail
The US withdrawal will become effective at the end of December 2018 - until then it would will remain a full member.
UNESCO's executive board plans to select her successor Friday in a secret ballot.
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 U.S. told Bokova it intends to stay engaged as a non-member "observer state" on "non-politicized" issues, including the protection of World Heritage sites, advocating for press freedoms and promoting scientific collaboration and education. "Ideally, it steers it in way that US interests and UNESCO's mandate will converge".
In July, the agency again delighted Palestinians when it declared the Old City of Hebron in the occupied West Bank an endangered World Heritage site.More news: Saudi king in historic first visit to Russian Federation for talks with Putin
Việt Nam's bid for the leading position of a respectable global organisation such as UNESCO has made evident its commitment to actively contribute to the worldwide community.