The United States and Israel have officially quit the United Nations Educational, Scientific and Cultural Organization (UNESCO) at the stroke of midnight on New Year's Day, the culmination of a process triggered more than a year ago.

The withdrawal is mainly procedural yet serves a new blow to UNESCO, co-founded by the US after World War II to foster peace.

The Trump administration filed its notice to withdraw in October 2017 and Israeli Prime Minister Benjamin Netanyahu followed suit, accusing the UN agency of anti-Israel bias.

The Paris-based organisation has previously criticised Israel's occupation of East Jerusalem and granted full membership to Palestine in 2011.

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The US has demanded "fundamental reform" in the agency.

UNESCO is best known for its work to preserve heritage, including maintaining a list of World Heritage sites, and programmes to promote education in developing countries.

The withdrawals will not greatly affect UNESCO financially, since it has been dealing with a funding slash ever since 2011 when both Israel and the US stopped paying dues after Palestine was voted in as a member state.

Since then officials estimate that the US - which accounted for around 22 percent of the total budget - has accrued $600m in unpaid dues, which was one of the reasons for President Donald Trump's decision to withdraw. Israel owes an estimated $10m.

Officials say that many of the reasons the US cited for the withdrawal do not apply any more, noting that since then, all 12 texts on the Middle East passed at UNESCO have been consensual among Israel and Arab member states.

The State Department couldn't comment because of the US government shutdown.

Earlier, the department told UNESCO officials the US intends to stay engaged at UNESCO as a non-member "observer state" on "non-politicised" issues, including the protection of World Heritage sites, advocating for press freedom and promoting scientific collaboration and education.

The US could potentially seek that status during the UNESCO Executive Board meetings in April.

The US has pulled out of UNESCO before. The Reagan administration did so in 1984 because it viewed the agency as mismanaged, corrupt, and used to advance Soviet interests. The US rejoined in 2003.