Rather than typing out a whole post I will snip a relevant section from a manuscript of mine, from the 'Roadmap' chapter:
Relying on existing mechanisms and on-the-ground resources, Quartet representatives begin informal monitoring and consult with the parties on establishment of a formal monitoring mechanism and its implementation. (My emphasis)
{text deleted} ... it is perhaps worth pointing out that a “formal monitoring mechanism” of any kind is consistently rejected by the United States, which regularly vetoes demands for them at the UN Security Council, votes against them at the UN General Assembly, refuses to attend intl. humanitarian gatherings where they are discussed etc
27 {continues}
The relevant reference:
27. Among many examples, the U.S. voted against General Assembly Resolution A/58/L.26 (Dec 3, 2003), put forward by the entire Arab League which called for “the implementation of the road map” whilst stressing “in this regard the importance and urgency of establishing a credible and effective third-party monitoring mechanism”, whilst condemning in the harshest terms “all acts of violence and terror against civilians on both sides”. The U.S. vetoed Security Council Resolution S/2001/270 (Mar 26, 2001), calling for a United Nations observer force (in the session preceding this vote, the Israeli representative Yehuda Lancry stated: “As we have stated before, Israel remains opposed to the establishment of a United Nations force in the region. Chairman Arafat has the ability to protect the lives of his people himself”). The U.S. vetoed Security Council Resolution S/2001/1199 (Dec 14, 2001) which encouraged the establishment of ”a monitoring mechanism” to help implement official U.S. recommendations (the Mitchell report); the same resolution also condemned “in particular” all “acts of violence and terror resulting in the deaths and injuries among Palestinian and Israeli civilians”. Immediately before the latter (Dec 5) the U.S. refused to attend a meeting of the High Contracting Parties to the 4th Geneva Conventions (attended by almost every U.S. ally, including Britain and the entire EU), during which intl. monitors and observers were specifically called for. See section 3.4.1, and sources cited for further discussion.