United Nations rights investigator Agnes Callamard has been appointed the new chief of Amnesty International.

Callamard will be secretary general of the international rights group effective immediately for a four-year term, Amnesty said in a statement Sunday night.

The French rights expert has worked as the UN's special rapporteur on extrajudicial, summary or arbitrary executions since 2016.

She has led investigations in more than 30 countries, including a high-profile UN inquiry into the 2018 killing of Saudi journalist Jamal Khashoggi at the Saudi consulate in Istanbul.

Sarah Beamish, chair of Amnesty's international board, praised Callamard's "intellectual acuity, her deep global human rights experience, and her courageous voice," in a statement announcing the appointment.

Callamard said she was honoured to take up the post.

"Where governments and corporations seek to silence those who speak out against their abuses, to obfuscate the truth, and to undermine or reject human rights norms, the rigorous investigations and uncompromising campaigns of Amnesty International are more vital than ever," Callamard said.

Callamard previously worked with Amnesty from 1995 to 2001. She takes over Amnesty's leadership post from Acting Secretary General Julie Verhaar.

Amnesty has offices in more than 70 countries and describes itself as the world's largest non-governmental human rights organization.