Omar al-Bashir, the Sudanese president, has siphoned as much as $9bn out of his impoverished country, and much of it may be stashed in London banks, according to secret US diplomatic cables that recount conversations with the chief prosecutor of the international criminal court.

Some of the funds may be held by the part-nationalised Lloyds Banking Group, according to prosecutor, Luis Moreno-Ocampo, who told US officials it was time to go public with the scale of Bashir's theft in order to turn Sudanese public opinion against him.

"Ocampo suggested if Bashir's stash of money were disclosed (he put the figure at $9bn), it would change Sudanese public opinion from him being a 'crusader' to that of a thief," one report by a senior US official states. "Ocampo reported Lloyds bank in London may be holding or knowledgeable of the whereabouts of his money," the cable says. "Ocampo suggested exposing Bashir had illegal accounts would be enough to turn the Sudanese against him."

Lloyds responded by saying it had no evidence of holding funds in Bashir's name. "We have absolutely no evidence to suggest there is any connection between Lloyds Banking Group and Mr Bashir. The group's policy is to abide by the legal and regulatory obligations in all jurisdictions in which we operate."

Details of the allegations emerge in the latest batch of leaked embassy cables released by WikiLeaks which reveal that:

• US officials regard European human rights standards as an "irritant", criticising the Council of Europe for its stance on secret rendition of terror suspects.

• Diplomats believe judges in the war crimes trial of the Liberian ex-president Charles Taylor have been deliberately causing delays to ensure the only African judge is presiding when the verdict is delivered.

The cables were released as the WikiLeaks editor-in-chief, Julian Assange, accused the US of mounting an aggressive, illegal investigation against him. "I would say that there is a very aggressive investigation, that a lot of face has been lost by some people, and some people have careers to make by pursuing famous cases, but that is actually something that needs monitoring," he told reporters outside the mansion on the Norfolk/Suffolk border where he is staying while on bail.

Assange has repeatedly asserted that he is the victim of a smear campaign. The Guardian today publishes the first full account of the allegations made against him by two Swedish women based on previously unseen police documents.

If Ocampo's claim about Bashir's fortune is correct, Sudanese funds being held in London banks amount to one tenth of annual GDP in Sudan, which ranks fifteenth from bottom in the UN's index of the world's poorest countries. Ocampo discussed evidence of the stash with the Americans just days after issuing an arrest warrant for the Sudanese president in March 2009, the first issued by the court against a serving head of state. Bashir was indicted for seven counts of war crimes and crimes against humanity last year with a further three counts of genocide added in July. Ocampo, who has never released details of the alleged funds, was severely criticised for the indictment by many in Sudan and internationally amid criticisms the move would inflame fighting in the southern Darfur region.

Despite the humanitarian crisis in Sudan, Bashir has remained popular among many others in the country, particularly those who have benefited from the oil boom brought about during his presidency. A spokesperson for the Sudanese government dismissed the claim, describing it as further evidence of the ICC's political agenda in discrediting the Sudanese government.

"To claim that the president can control the treasury and take money to put into his own accounts is ludicrous – it is a laughable claim by the ICC prosecutor," said Dr Khalid al-Mubarak, government spokesperson at the Sudanese embassy in London. "Ocampo is a maverick, and this is just part of his political agenda. He has failed miserably in all his cases and has refused to investigate Iraq or Gaza – he needs success and he has targeted Bashir to increase his own importance."

"Attempts to smear not only Bashir but Sudan as a whole are well known, and are clearly linked with anti-Arab sentiments and Islamophobia," Mubarak added.

But experts said that if confirmed, the funds could have big implications for victims of human rights abuses in the county. Richard Dicker, head of international justice at Human Rights Watch, said: "If Bashir were to be tried and convicted, these funds could not just be frozen, but used as a source of reparations for victims … [of] horrific crimes in Darfur."

Robert Palmer, a campaigner at anti-corruption organisation Global Witness, said: "$9bn may sound like an inconceivably large amount of money for the president of Sudan to control. But we have uncovered evidence of substantial funds being held in a European bank by an oil-rich country in the past, where the head of state had a worrying level of personal control over the funds. In Sudan's case, the figure is almost the same amount as has been transferred from north to south Sudan under the oil revenue sharing part of the comprehensive peace agreement since 2005."

In a remarkable series of exchanges, the cables also reveal how Sudan's mineral wealth had a direct bearing on the ICC proceedings against Bashir, as China balked at action against him that could harm its interests in the oil industry. "Ocampo said China, as long as it continues to have oil concessions in Sudan, does not care what happens to Bashir," one cable states.

In another cable dated March 2008, a senior French official noted "growing Chinese concern about possible north-south fissures in Sudan and the possibility that its oil interests could be threatened".

"The Chinese were beginning to see more clearly that Sudan's behaviour towards Darfur and Chad could only increase the possibility of a north-south rupture will a possibly severe effect on China's stake in the oil sector,' the French are reported to have said.

In return, the Chinese expressed "puzzlement" that the French – a member of the ICC and able to influence the deferral of proceedings against Bashir – supported Ocampo's decision to pursue the Sudanese president, given France's oil interests in the region. "[The Chinese] observed French companies have oil interests in Sudan as well as Chad," the Americans stated.

France ultimately supported Bashir's indictment, but the cables suggest this was deliberately calculated to protect their oil interests. The French told the Americans they believed that firm action on Darfur was the only way to protect oil interests.

Both French firm Total and China, through affiliates of its state-owned China Petroleum and Chemical Corporation, have substantial oil concessions in Sudan, which currently produces 500,000 barrels of crude oil per day .

"It is ironic that China, which postures as a friend of the people's in the developing world couldn't give a damn about the suffering of hundreds of thousands of African victims in Darfur," said Dicker. "I'm not surprised that China is putting its oil interests above the interests of humanity in seeing that these crimes of enormous concern are adjudicated, but I think it will rebound to China's discredit," Dicker added.Speculation that Bashir may have deposited billions in oil money in foreign accounts is likely to add to demands for his arrest and transparency in Sudan's oil sector."The arm of the law, when it comes to this type of crime, committed by or alleged to have been committed by heads of state or heads of government, has gotten longer," said Dicker. "There is a long road to trial in The Hague, but what's striking is a number of other heads of state and heads of government have wound up in court much to their surprise through often lengthy and circuitous pathways."