War crimes judges have upheld the genocide conviction of former Bosnian Serb military chief Ratko Mladic over the 1995 Srebrenica massacre, Europe's worst act of bloodshed since World War II.

The United Nations tribunal in The Hague rejected his appeal against his 2017 life sentence for genocide, war crimes and crimes against humanity over the 1992-5 Bosnian war.

Dubbed the "Butcher of Bosnia", the once burly general who is now in his late 70s sat impassively and listened to the judgment through headphones as it was read out by presiding judge Prisca Nyambe.

"The appeals chamber affirmed the sentence of life imprisonment imposed on Mr Mladic by the trial chamber," the tribunal said in a statement.

The verdict by five judges at the UN International Residual Mechanism for Criminal Tribunals, which deals with cases from the now-closed Yugoslavia war crimes tribunal, is final and cannot be appealed any further.

Mothers of some of the 8,000 mostly Muslim men and boys killed in cold blood when Bosnian Serb troops overran Srebrenica were outside the court where they have long campaigned for justice.

"Today is a historic day, not only for us mothers, but also for the whole Balkans, Europe and the world," Munira Subasic, president of one of the "Mothers of Srebrenica" associations, said outside court.

"He's a monster who did not repent for what he has done, even after 26 years. Wherever their army came, wherever their boot stepped in, they committed genocide," she added.

8,000 men and boys who lost their lives are commemorated in Srebrenica

Prosecutors had also appealed against Mladic's acquittal on wider genocide charges.

Earlier, tribunal prosecutor Serge Brammertz had said he was "cautiously optimistic" about the verdict, with the Belgian jurist telling reporters last week he could not "imagine another outcome than confirmation" of at least the original verdict.

Mladic, who spent a decade on the run before his capture in 2011, was the military face of a brutal trio led on the political side by ex-Yugoslav president Slobodan Milosevic and former Bosnian Serb leader Radovan Karadzic.

Mladic was found guilty of genocide for personally overseeing the massacre at the supposedly UN-protected enclave of Srebrenica as part of a campaign to drive out Muslims.

Mladic with Bosnian Serb leader Radovan Karadzic, who is serving a life sentence for genocide in Srebrenica

Footage from the time showed him handing out sweets to children before they and the women of the town were taken away by bus, while the men were marched into a forest and executed.

He was also found guilty of orchestrating a wider campaign of "ethnic cleansing" to drive Muslims and Bosnians out of key areas to create a Greater Serbia as Yugoslavia tore itself apart after the fall of communism.

The war left around 100,000 people dead and 2.2 million displaced.

But Mladic, who gives his age as 78 but it is 79 according to the court, insisted during an appeal hearing last year that "fate put me in a position to defend my country".

During a long tirade, he also said he was a "target of the NATO alliance" and derided the court as a "child of western powers".

The appeal hearing was delayed repeatedly after Mladic needed surgery to remove a polyp, and then because of the Covid-19 pandemic. Access to the court today was also limited because of coronavirus measures.

Mladic is the last of the Serb trio to face justice, with Milosevic dying of a heart attack in his cell in The Hague in 2006 before his trial had finished, while Karadzic is serving a life sentence for genocide in Srebrenica.

Relatives of the victims hoped that the court would also overturn Mladic's acquittal on wider genocide convictions, saying it was necessary for reconciliation between still-divided communities.

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United Nations High Commissioner for Human Rights Michelle Bachelet welcomed the tribunal ruling, saying it "highlights the determination of the international justice system to ensure accountability no matter how long it may take - in Mladic's case, nearly three decades after he committed his appalling crimes".

Her comments came in a joint statement with Alice Wairimu Nderitu, special adviser to the UN Secretary-General on prevention of genocide.

"The decision to uphold Mladic's convictions by the International Tribunal for the Former Yugoslavia, as well as his sentence of life imprisonment, provides historical certainty and finality for victims and survivors," Ms Nderitu said.

She added that the outcome sent "a hugely important message throughout the Western Balkans" where "genocide denial and the glorification of convicted criminals such as Mladic (is) not only persisting but increasing".

US President Joe Biden said the "historic judgment shows that those who commit horrific crimes will be held accountable" and "reinforces our shared resolve to prevent future atrocities from occurring anywhere in the world".