Human rights organisations have cast doubt on claims of mass rape and other abuses perpetrated by forces loyal to Colonel Muammar Gaddafi, which have been widely used to justify Nato's war in Libya.

Nato leaders, opposition groups and the media have produced a stream of stories since the start of the insurrection on 15 February, claiming the Gaddafi regime has ordered mass rapes, used foreign mercenaries and employed helicopters against civilian protesters.

An investigation by Amnesty International has failed to find evidence for these human rights violations and in many cases has discredited or cast doubt on them. It also found indications that on several occasions the rebels in Benghazi appeared to have knowingly made false claims or manufactured evidence.

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The findings by the investigators appear to be at odds with the views of the prosecutor of the International Criminal Court, Luis Moreno-Ocampo, who two weeks ago told a press conference that "we have information that there was a policy to rape in Libya those who were against the government. Apparently he [Colonel Gaddafi] used it to punish people."

US Secretary of State Hillary Clinton last week said she was "deeply concerned" that Gaddafi's troops were participating in widespread rape in Libya. "Rape, physical intimidation, sexual harassment, and even so-called 'virginity tests' have taken place in countries throughout the region," she said.

Donatella Rovera, senior crisis response adviser for Amnesty, who was in Libya for three months after the start of the uprising, says that "we have not found any evidence or a single victim of rape or a doctor who knew about somebody being raped".

She stresses this does not prove that mass rape did not occur but there is no evidence to show that it did. Liesel Gerntholtz, head of women's rights at Human Rights Watch, which also investigated the charge of mass rape, said: "We have not been able to find evidence."

In one instance two captured pro-Gaddafi soldiers presented to the international media by the rebels claimed their officers, and later themselves, had raped a family with four daughters. Ms Rovera says that when she and a colleague, both fluent in Arabic, interviewed the two detainees, one 17 years old and one 21, alone and in separate rooms, they changed their stories and gave differing accounts of what had happened. "They both said they had not participated in the rape and just heard about it," she said. "They told different stories about whether or not the girls' hands were tied, whether their parents were present and about how they were dressed."

Seemingly the strongest evidence for mass rape appeared to come from a Libyan psychologist, Dr Seham Sergewa, who says she distributed 70,000 questionnaires in rebel-controlled areas and along the Tunisian border, of which over 60,000 were returned. Some 259 women volunteered that they had been raped, of whom Dr Sergewa said she interviewed 140 victims.

Asked by Diana Eltahawy, Amnesty International's specialist on Libya, if it would be possible to meet any of these women, Dr Sergewa replied that "she had lost contact with them" and was unable to provide documentary evidence.

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The accusation that Viagra had been distributed to Gaddafi's troops to encourage them to rape women in rebel areas first surfaced in March after Nato had destroyed tanks advancing on Benghazi. Ms Rovera says that rebels dealing with the foreign media in Benghazi started showing journalists packets of Viagra, claiming they came from burned-out tanks, though it is unclear why the packets were not charred.

Credible evidence of rape came when Eman al-Obeidy burst into a hotel in Tripoli on 26 March to tell journalists she had been gang-raped before being dragged away by the Libyan security services.

Rebels have repeatedly charged that mercenary troops from Central and West Africa have been used against them. The Amnesty investigation found there was no evidence for this. "Those shown to journalists as foreign mercenaries were later quietly released," says Ms Rovera. "Most were sub-Saharan migrants working in Libya without documents."

Others were not so lucky and were lynched or executed. Ms Rovera found two bodies of migrants in the Benghazi morgue and others were dumped on the outskirts of the city. She says: "The politicians kept talking about mercenaries, which inflamed public opinion and the myth has continued because they were released without publicity."

Nato intervention started on 19 March with air attacks to protect people in Benghazi from massacre by advancing pro-Gaddafi troops. There is no doubt that civilians did expect to be killed after threats of vengeance from Gaddafi. During the first days of the uprising in eastern Libya, security forces shot and killed demonstrators and people attending their funerals, but there is no proof of mass killing of civilians on the scale of Syria or Yemen.

Most of the fighting during the first days of the uprising was in Benghazi, where 100 to 110 people were killed, and the city of Baida to the east, where 59 to 64 were killed, says Amnesty. Most of these were probably protesters, though some may have obtained weapons.

Amateur videos show some captured Gaddafi supporters being shot dead and eight badly charred bodies were found in the remains of the military headquarters in Benghazi, which may be those of local boys who disappeared at that time.

There is no evidence that aircraft or heavy anti-aircraft machine guns were used against crowds. Spent cartridges picked up after protesters were shot at came from Kalashnikovs or similar calibre weapons.

The Amnesty findings confirm a recent report by the authoritative International Crisis Group, which found that while the Gaddafi regime had a history of brutally repressing opponents, there was no question of "genocide".

The report adds that "much Western media coverage has from the outset presented a very one-sided view of the logic of events, portraying the protest movement as entirely peaceful and repeatedly suggesting that the regime's security forces were unaccountably massacring unarmed demonstrators who presented no security challenge".

The rising cost of war

The Nato-led air campaign in Libya will cost the UK at least £260m if it continues for another three months, Defence Secretary Liam Fox said yesterday.

The estimate stands in sharp contrast to the figures predicted by George Osborne in March, when the Chancellor said Britain's involvement would be likely to cost "tens of millions, not hundreds of millions" of pounds.

Mr Fox told Parliament that the projected cost was "in the region" of £120m, with an additional £140m bill to replace missiles and other weapons. He said attempts to minimise civilian casualties had led to a steeper bill.

Emily Fairbairn