Ban Ki Moon the Secretary-General of the United Nations, visited Sri Lanka last week. He knew from his officials that at least 20,000 civilians had been killed by Sri Lankan troops in the offensive against the Tamil Tigers. Mr Ban never mentioned this figure to his Sri Lankan interlocutors. He saw, while travelling by air over a supposed “no-fire” zone, the evidence of a massacre of thousands of Tamil civilians caught between the army and the insurgents. Yet he has still not confirmed the authenticity of photographs taken from the same helicopter setting out that scene of carnage and mass makeshift graves.

There is a terrible augury for such inexplicable reticence. The day after Bosnian Serb forces seized Srebrenica, deemed by the United Nations to be a “safe area”, in 1995, Boutros Boutros Ghali, Mr Ban’s predecessor, was asked whether this represented the organisation’s greatest failure in Bosnia. He replied: “No, I don’t believe this represents a failure. You have to see if the glass is half full or half empty.”

The name of Srebrenica, in which 8,000 Bosniak men and boys were slaughtered, has become synonymous with insouciance and failure by the UN, and not only with the barbarism of the perpetrators.

Any parallel for the UN with what has happened in Sri Lanka must be scotched now. That can be done only by Mr Ban speaking forthrightly about what he saw. He is a civil servant rather than an executive; and the cause of historical truth as well as international protest depends on the UN Security Council’s having full and public knowledge of what he saw.

There is no case for restricting diplomacy to private channels. There is no confidential quality to what Mr Ban can testify. David Miliband, the British Foreign Secretary, was denied access to the north of the country when he visited Colombo a fortnight ago. He has a belated but important role now in pressing Mr Ban to speak.

There is no question but that the defeated Tamil Tigers were guilty of numerous depredations and horrific acts of suicide terrorism. Vellupillai Prabhakaran, the slain leader of the Tigers, denied, among other murderous acts, sending the assassin of Rajiv Gandhi, the former Indian Prime Minister; few doubt that he was lying. But the shelling of civilians in a supposed safe area and their deaths by the tens of thousands are not a matter for the Sri Lankan Government alone. UN sources have described the offensive as a war waged without witnesses. However just the cause and brutal the adversary, there must always be disinterested witnesses to military campaigns. That is particularly so with adversaries that practise indiscriminate attacks: there is always a temptation, in response, to suspend the laws of war and the observance of due process in the name of a higher necessity. And that temptation must always be resisted.

The Sri Lankan Government has much to account for. Yet it has responded with disingenuity and fantasy. It first denied the deaths of civilians and then claimed that the photographic evidence, repeated by independent witnesses, had been forged. In doing so, it is perpetrating sins of omission in order to obscure those of commission. Mr Ban must speak; the UN must investigate. Nothing else will demonstrate a decent respect for the opinions of mankind.