The Nobel peace prize has been awarded to Navi Pillay, the South African former UN human rights chief who chaired the UN commission that concluded Israel was guilty of genocide in Gaza.

The Norwegian Nobel committee said Pillay, who is serving as a judge at the international court of justice (ICJ), has “shown that legal measures can help prevent acts of war and violence”.

“Navi Pillay’s commitment to universal legal principles and her firm moral compass are constants in a long career,” the committee said, mentioning her early work defending Nelson Mandela under South Africa’s apartheid regime, serving on the international criminal tribunal, and then as the UN high commissioner for human rights, as well as chairing the UN independent international commission of inquiry on the occupied Palestinian territory.

That commission concluded last year that the state of Israel was guilty of genocidal acts in Gaza “with the specific intent to destroy Palestinians in Gaza”, and said the country’s prime minister, Benjamin Netanyahu, its president, Isaac Herzog, and its former defence minister Yoav Gallant had “incited the commission of genocide”.

“It is clear that there is an intent to destroy the Palestinians in Gaza through acts that meet the criteria set forth in the genocide convention,” Pillay said on the publication of the commission’s findings in September last year.

Israel refused to cooperate with the commission, arguing it was biased, and rejected its report as “scandalous”. The Israeli foreign ministry reacted furiously to news of Pillay’s award on Friday.

“If there were an award for lies, hatred, and blood libels, Navi Pillay wouldn’t just be a contender. She’d be a frontrunner,” the ministry said on X, adding: “Awarding Pillay the Nobel Peace Prize isn’t a recognition of justice. It’s the grotesque weaponization of an award to legitimize anti-Israel hatred and prejudice.

“Now that the Nobel commission has rewarded the biggest lie of the 21st century, Joseph Goebbels is certainly applauding from hell.”

In its statement, the Nobel committee said Pillay, 85, had “contributed to strengthen the institutions of international law”.

“Her independence, expertise and steadfastness have made her one of the most respected international jurists of our time,” the committee said.

“In awarding this year’s Nobel peace prize, the Norwegian Nobel committee wants to remind the world that the rule of law – national and international – underpins the system we have set up to ensure peace and resolve conflicts non-violently,” its statement said.

The committee said it had not been able to locate Pillay before its announcement on Friday morning. She was attending a meeting of human rights lawyers and jurists in Nuremberg, Germany, at the site of postwar war crimes trials of Nazi leaders, and heard the news from colleagues. She responded with disbelief.

“I don’t know about it so much. The question is: is it true? Being a lawyer and a judge, I still want to verify it,” she said.

“The last thing I did was the report on Gaza naming what’s happening there as genocide,” Pillay added. “One of the main reactions to that report was people [complimented] me for my courage. You can imagine how surprised I was. Why would you need courage to speak the truth. You interpret the law. You look at all the facts, so it’s an impartial decision.”

Pillay is currently overseeing a genocide case against Myanmar at the ICJ.

The ICJ, a UN body, rules on disputes between states, while the international criminal court (ICC) investigates and prosecutes individuals accused of atrocities and other grave international crimes.

Hours after the announcement, the Trump administration unveiled broad new sanctions against the ICC, on which Pillay also once served. The US secretary of state, Marco Rubio, said the measures were to ensure the court would never prosecute Americans.

The ICC has not charged any US nationals, and has previously pointed out that it is a court of last resort, unnecessary in the case of countries with effective legal systems. Last year, Donald Trump, who has long coveted the Nobel peace prize, imposed personal sanctions against ICC officials deemed to be involved in investigations of Americans or Israelis. The new sanctions target the court as an institution, blocking any ICC assets in the US and giving US companies and individuals six months to wind down any business with the court.

The ICC said the sanctions were “an attempt to obstruct the course of justice and to prevent the court from doing what it was created to do”.

“It is an assault on the rule of law and on the very foundations of the international legal order which strikes at the simple principle that no one stands above the law,” the court said in a statement.

Pillay’s award was welcomed by human rights activists and hailed in South Africa, home to four previous laureates. The country’s president, Cyril Ramaphosa, said: “Justice Pillay’s crowning as winner of the Nobel prize allows us to celebrate the progressive contributions she has made to strengthen peace, uphold human dignity and bring hope in South Africa and to peoples globally.”

Edwin Cameron, a retired South African constitutional court justice, said: “The award of the prize to Navi Pillay re-emphasises that the prize embodies an international aspiration to peace, and to respect and understanding across borders – and that leaders of great stature in our country – [Desmond] Tutu, Mandela, now Pillay – are recognised outside it, on the international stage.

“This may help reset our national debate on South Africa’s position in the region, and in Africa,” he said.

Yasmin Sooka, a human rights lawyer and commissioner on both South Africa and Sierra Leone’s truth and reconciliation commissions, said Pillay “follows in the footsteps of Chief Albert Luthuli and Archbishop Desmond Tutu, carrying forward South Africa’s proud tradition of linking peace with justice and human dignity”.

“From defending apartheid’s most vulnerable to her pioneering role in the Akayesu judgment, she has transformed international justice. That judgment helped establish rape and sexual violence as weapons of war – a historic breakthrough that placed survivors’ experiences at the heart of international justice,” Sooka said.

In 1998, the international criminal tribunal for Rwanda found Jean-Paul Akayesu, a former mayor of Taba, guilty of genocide and crimes against humanity.

“At the ICC, as UN high commissioner for human rights, and leading the inquiry that found Israel responsible for genocide in Gaza, she has brought unwavering integrity and courage to holding power accountable,” Sooka added.

Last year’s award of the peace prize to the Venezuelan opposition leader, María Corina Machado, drew widespread criticism because of her calls for US military intervention in her country. Machado heightened the controversy in January by handing her winner’s medal to Trump. The prize committee insisted that the honour of being a Nobel laureate could not be transferred.

On Friday, Pillay joked that she could do the same: “If you clap a little bit more, I’m going to share this prize with Trump.”