"This decision was not taken lightly, and reflects U.S. 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.
Earlier on Thursday, the United States formally notified the United Nations Educational, Scientific and Cultural Organization (UNESCO) that it would withdraw from the organization on December 31, 2018.
Leaving UNESCO might seem like typical Trump isolationism, but the U.S.'s beef with the organization goes back to previous administrations.
Heather Nauert, US state department spokesperson, said on Thursday the US would establish an "observer mission" to replace its representation at the Paris-based agency.
The United States has walked away once before under ex-president Ronald Reagan, who quit UNESCO in 1984 over alleged financial mismanagement and claims of anti-US bias in some of its policies.
Israel on Thursday welcomed USA pulling out of the UN's cultural organization, announcing the Jewish state was considering a similar move due to the body's "anti-Israel" bias.
"The US relationship with Unesco is a matter for them".
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Heynckes added: "I wouldn't have returned to any other club in the world, but Bayern Munich is a love affair for me. Work in Spain followed, at Athletic Bilbao, Tenerife and Real Madrid, where he won the Champions League in 1997.
Still, she noted the Trump administration wants the U.S.to remain engaged with the United Nations agency "as a non-member observer state in order to contribute USA views, perspectives and expertise on some of the important issues undertaken by the organization, including the protection of world heritage, advocating for press freedoms, and promoting scientific collaboration and education".
Tensions have flared again in recent years: in 2011, the United States stopped funding the agency after UNESCO voted to admit the Palestinian Authority as a full member; two years later, the U.S. lost its vote due to these financial cuts.
"Universality is critical to UNESCO's mission to strengthen global peace and security in the face of hatred and violence, to defend human rights and dignity", she wrote.
"At the time when conflicts continue to tear apart societies across the world, it is deeply regrettable for the United States to withdraw from the United Nations agency promoting education for peace and protecting culture under attack", the statement said.
Israel has been irked by resolutions that diminish its historical connection to the Holy Land and have instead named ancient Jewish sites as Palestinian heritage sites.
He said he has ordered Israeli diplomats to prepare Israel's withdrawal from the organisation in concert with the Americans. Permanent Representative to the UN Nikki Haley has repeatedly denounced these actions, and said the US was "evaluating" whether or not it should continue as a member of the organization. "This is a loss for multilateralism". After he was elected but before he became president, Mr. Trump made an extraordinary intervention on the world stage by criticizing the Obama administration's decision not to block a United Nations resolution criticizing Israeli settlements.
The U.S. joined UNESCO at its creation in 1945. The Reagan administration withdrew from the organisation in 1984 - accusing the agency of corruption and an ideological bias towards the then Soviet Union - but the USA rejoined in 2002.