U.N. Secretary-General Ban Ki-moon warned world powers on Thursday not to repeat in Syria the mistakes they made in Bosnia, during a landmark visit to Srebrenica where U.N. peacekeepers failed to prevent the massacre of 8,000 Muslim men and boys.

The United Nations had designated the enclave in eastern Bosnia a "safe haven" for Muslim refugees, but peacekeepers stood by helplessly as Bosnian Serb forces carried out the slaughter in 1995.

It followed three years of hand-wringing and hesitation by divided world powers reluctant to intervene in the 1992-95 Bosnian war. About 100,000 people died before they finally did.

During the conflict in Syria, the U.N. Security Council, split between Western powers on one side and Russia and China on the other, has proved similarly helpless.

"I don't want to see any of my successors after 20 years visiting Syria and apologising for what we could have done now to protect civilians in Syria, which we are not doing," Ban said after laying flowers at a white marble memorial to the Srebrenica victims.

"Never Srebrenica," he said, "Nowhere, to nobody."

The visit - the first by a head of the United Nations to Srebrenica - ended a week-long tour by Ban of the countries carved from old federal Yugoslavia.

On Wednesday, he told the Bosnian parliament in Sarajevo, where 10,000 people died in a 43-month siege, that he was making a plea to the world to unite and "stop the slaughter" in Syria.

Ban's predecessor Kofi Annan was head of U.N. peacekeeping operations at the time of the Srebrenica massacre and is the U.N and Arab League envoy to Syria, tasked with finding a political solution to the violence.

Still Burying The Dead

Activists say at least 18,000 people have been killed in the 16-month-old Syrian uprising against President Bashar al-Assad, whose forces on Thursday shelled parts of the capital Damascus and second city, Aleppo.

The Srebrenica massacre was the worst mass killing on European soil since World War Two. Bosnia is still burying the dead, 5,657 so far, under row upon row of white tombstones. The remains of some 2,400 more have still to be identified or dug up from mass graves in hilly eastern Bosnia.

Bosnian Serb wartime commander Ratko Mladic and his political leader, Radovan Karadzic, are standing trial at the U.N. war crimes tribunal in The Hague accused of genocide in Srebrenica and other crimes.

Relatives of victims have tried to take the United Nations to court for its blue-helmeted peacekeepers, who were from the Netherlands, failing to prevent the massacre.

In April, the Dutch Supreme Court ruled that the United Nations could not be prosecuted in the Netherlands over the massacre, and the relatives said they would appeal to the European Court of Human Rights.

"The United Nations must face up to its responsibility," said Hasan Nuhanovic, a Bosnian Muslim who worked as a U.N. interpreter during the war. His mother, father and brother were killed.

"The people who surrendered my family to Serb forces wore blue helmets," he said.