US lawyers claim they have videos implicating Abu Dhabi royal in more cases of torture, a week after outcry over his assaults on Afghan businessman.

The wealthy Gulf prince at the centre of a "torture tape" scandal has been accused of attacking at least 25 other people in incidents that have also been caught on film, it has been claimed.

Sheikh Issa bin Zayed al-Nahyan is now under investigation in the United Arab Emirates after the shocking tape showed him beating a man with a nailed plank, setting him on fire, attacking him with a cattle prod and running him over.

But now lawyers for American businessman Bassam Nabulsi, who smuggled the tape out of the UAE, have written to the justice minister of Abu Dhabi - the most powerful of the emirates that make up the UAE - claiming to have considerably more evidence against Issa.

"I have more than two hours of video footage showing Sheikh Issa's involvement in the torture of more than 25 people," wrote Texas-based lawyer Anthony Buzbee in a letter obtained by the Observer.

The news of more torture videos involving Issa is another huge blow to the international image of the UAE. The oil-rich state has been keen to develop relations with wealthy western politicians, universities and corporations and to promote an aura of moderation and tolerance. But the shocking video of Issa torturing Afghan grain merchant Mohammed Shah Poor, whom Issa said had cheated him in a business deal, has heavily dented the UAE's reputation. Particularly damaging was the apparent involvement of a policeman in the torture and the impunity with which Sheikh Issa could act, even after the tape emerged. He is a senior prince related to powerful members of the ruling family in Abu Dhabi.

But now it appears the initial tape could just be the beginning of the problem. The new tapes apparently also involve police officers taking part in Issa's attacks, and some of his victims in the as-yet-unseen videos are believed to be Sudanese immigrants.

Buzbee said he would be happy to provide the new videos to the Abu Dhabi authorities, who have pledged to investigate Issa's activities. "I also have access to at least three witnesses, all of whom will testify that the brutality exhibited in the videos by Sheikh Issa is part of a pattern of conduct that has gone on for some time ... I can also provide additional witnesses who were actually present during several of Sheikh Issa's torture sessions," Buzbee said in the letter.

The fresh revelations about Issa's actions will add further doubt to a pending nuclear energy deal between the UAE and the US. The deal, signed in the final days of George W Bush, is seen as vital for the UAE. It will see the US share nuclear energy expertise, fuel and technology in return for a promise to abide by non-proliferation agreements. But the deal needs to be recertified by the Obama administration and there is growing outrage in America over the tapes. Congressman James McGovern, a senior Democrat, has demanded that Hillary Clinton, the US secretary of state, investigate the matter and find out why US officials initially appeared to play down its significance.

The authorities in the UAE have certainly mishandled the emergence of the initial torture tape. The 2004 tape was obtained by ABC News and shown on television in the US. The UAE at first said that the matter had been privately settled between Sheikh Issa and his victim. They also added that UAE police had followed all their rules and regulations properly.

But that position did not last long in the face of a wave of international revulsion at the brutality on display. The fierceness of the criticism eventually forced the UAE government to both condemn the tape and announce a new investigation. The government "unequivocally condemns the actions depicted on the video", the state-run news agency said last week. It added that a government human rights group in the Judicial Department would also now review the matter. Local media also started to report the existence of the tape, having hitherto been silent on the story in the notoriously secretive country.

Buzbee welcomed the developments, but expressed scepticism that the investigation was genuinely motivated, because the authorities had known about the tapes for several years. "I am sceptical about whether there will be a genuine investigation, given that various officials have been aware of these issues for many years and given the fact that members of the government were actually involved in, or covered up, the torture," he said.

The tape emerged from a court case brought in America by Nabulsi. The American citizen is a former business partner of Sheikh Issa, and claims he, too, was tortured in the UAE after the pair fell out. Nabulsi said the first tape was shot by his brother on the orders of Sheikh Issa, who liked to view them later for his own pleasure.

Lawyers for Issa in America have attacked Nabulsi's motivations, accusing him of waging a media campaign. In a statement last week, Issa's lawyer, Daryl Bristow, said: "When all the facts are known, the one-sided story being told by Nabulsi will be completely addressed and Nabulsi will be discredited."