The US Assistant Secretary of State for Israeli-Palestinian Affairs, Andrew Miller, has resigned, becoming the most senior US official to leave his post since the outbreak of the Gaza war last October.

Although Miller told colleagues on Friday that he was stepping down to spend more time with his family, a Washington Post article said his decision was a protest against the fact that the U.S. has allowed Israel to kill tens of thousands of people in Gaza.

American newspapers described Miller’s resignation as “a setback for U.S. diplomats seeking to break further with Israeli Prime Minister Benjamin Netanyahu and his far-right coalition”.

Miller’s resignation comes amid growing frustration inside and outside the U.S. government over the high number of civilians killed in Gaza, which has topped 37,000 so far, according to Gaza health authorities.

According to the Washington Post, Miller is described as “a principled supporter of Palestinian rights and statehood, as well as a meticulous thinker about the Middle East”.

Before devoting himself to the Israeli-Palestinian conflict, Miller served as political advisor to the U.S. ambassador to the United Nations and, under former President Barack Obama’s administration, as director of military affairs for Egypt and Israel at the White House’s National Security Council.

According to the American newspapers, Miller had warned against what he called the “bear hug” referring to Biden’s embrace of Netanyahu during the former’s visit to Tel Aviv, in the days following the Hamas attack.

Miller is believed to have believed that the leverage the United States has over Israel, as its largest military, economic and political supporter, could have been used more effectively to prevent civilian deaths in Gaza.