The chief prosecutor for the International Criminal Court said yesterday that he is investigating claims that Libyan leader Muammar Gaddafi provided Viagra to Libyan soldiers to promote the rape of women during the current conflict.

Luis Moreno Ocampo said his office was collecting evidence on rapes and had become "more convinced" that Gaddafi decided to punish women by using rape as a weapon, which would be a new method in the Libyan civil war of instilling fear and attempting to control the population.

He said some witnesses confirmed the Libyan government was buying containers of Viagra-type drugs to carry out the policy, and "to enhance the possibility to rape".

"We are trying to see who was involved," he said. Mr Moreno Ocampo said it was difficult to know how widespread the use of rape was in Libya.

He added: "In some areas we had a number of a hundred people raped.

"The issue for us was, can we attribute these rapes to Gaddafi himself, or is it something that happened in the barracks."

Meanwhile, thousands of troops loyal to Gaddafi - who has promised once more to fight to the death - advanced on the rebel-held western city of Misrata yesterday, shelling it from three sides in attacks that killed at least 12 rebels.

Nato airstrikes have targeted Muammar Gaddafi's sprawling Tripoli compound in a new round of daylight raids.

Nato's top official said Gaddafi's fall was only a matter of time.

British officials stated yesterday that UK and Nato forces would be prepared to turn their guns on their present allies, Libya's rebels, if they attacked civilians loyal to Muammar Gaddafi's regime.

The warning follows a report by Human Rights Watch accusing the opposition of abusing civilians.

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On Tuesday, Libyan state television broadcast an audio address from Gaddafi, who denounced Nato and the rebels challenging his rule. He vowed never to surrender. "We will not kneel!" he shouted. It could not be confirmed whether Gaddafi's speech was a live phone call or an audio recording, but it appeared to take state television by surprise. "We will not surrender," Gaddafi said.

Belfast Telegraph