Prime Minister Hun Sen told UN Secretary-General Ban Ki-moon he wanted the office head in Cambodia to be sacked.
The Cambodian government wants the United Nations to close its human rights office in Phnom Penh.
Ou Virak is on a two-week trip to the US, where he met with senior State Department officials and representatives of Congress.
Ban said he felt the government was “committed to completing the process” of the court and that Cambodians seek and deserve justice.
Soun Sophoan, who attempted to lead 30 activists toward Ban's motorcade, remained in serious condition on Thursday night.
Pung Chiv Kek, founder of the rights group Licadho, said Cambodia must appreciate the meaning of the Paris Peace Agreement of 1991.
Rights groups and other residents have looked for him to address what they say is a deteriorating rights situation in Cambodia.
Demonstrations, to be held in France, Canada and the US, will also coincide with the anniversary of the Paris Peace Agreement.
16 former staff members of the Cambodian Center for Human Rights alleges they were cheated out of their salaries and per diem payments.
The UN may be pushing for more funding at the tribunal, which has struggled financially all year.