TRIPOLI (AFP) - Libya's regime had its back to the wall on Wednesday as it faced NATO air strikes, the apparent defection of its oil minister and threats of prosecution by the International Criminal Court. Rebels fighting to topple strongman Moamer Gaddafi's regime by contrast were growing in confidence and laid claim on Wednesday to being able to represent Libya at the June 8 meeting of oil cartel OPEC in Vienna. "We want to attend, and will study the legal procedure," Mahmud Shammam, media spokesman for the rebel National Transitional Council (NTC), told AFP in Dubai. "We still do not know if OPEC will invite us," he said. Oil Minister Shukri Ghanem, a veteran of Gaddafi's regime, at the weekend crossed from Libya into neighbouring Tunisia, a Tunisian official said, although there has been no confirmation he has defected. Ghanem, also chairman of Libya's national oil company, had been due to attend the Vienna meeting of the Organisation of Petroleum Exporting Countries (OPEC) on behalf of the regime but he has made no comment since he left Libya and his whereabouts are not clear. Shammam said he believed the minister was already in the Austrian capital. "We have got confirmation from several sources that Shukri Ghanem is in his house in Vienna," he said. "He has not been in touch with the NTC, and the council is not interested, nor in need for Mr Ghanem, but we welcome defection by any official." If confirmed, Ghanem would be among the most senior officials to abandon Gaddafi's government since an uprising erupted in mid-February. In April, the US Treasury Department froze the assets of five senior Gaddafi regime figures, including Ghanem, in a bid to fracture the veteran ruler's inner circle. Libya is a key crude-exporting nation but its output has been slashed since the revolt began. Meanwhile, the International Criminal Court's prosecutor warned the Libyan regime on Wednesday it could face investigation and prosecution if it tries to cover up crimes committed against its people. The office of the prosecutor sent a letter to Libyan Foreign Minister Abdelati Laabidi, its head of jurisdiction Phakiso Mochochoko told AFP in The Hague. "The office calls upon you and other Libyan authorities to refrain from being involved in such cover up. Failure to do so will result in investigation and prosecution," said the letter, shown to AFP. The office "considers that part of the criminal plan implemented in Libya includes the cover-up of the crimes" committed to quash a popular uprising against Gaddafi that erupted in February. ICC chief prosecutor Luis Moreno-Ocampo has asked the court's judges to issue arrest warrants against Gaddafi, his second-oldest son Seif al-Islam and his brother-in-law Abdullah al-Senussi - for crimes against humanity. Asked how crimes were covered up, Moreno-Ocampo said: "Even Mr Gaddafi himself said 'where's the bodies?', because what they do is that their doctors are prohibited to register dead people in hospitals... the bodies are hidden." Libya's government spokesman Mussa Ibrahim has dismissed the ICC's bid, saying the court has no jurisdiction over Tripoli while denying accusations that the regime ordered the killing of civilians or hired mercenaries against them. Thousands of people have died in violent clashes pitting regime opponents and Gaddafi loyalists, and forced some 750,000 to flee, according to data from the ICC and the UN. Libya has been targeted almost daily by NATO-led strikes that began on March 19 after a UN resolution mandated a no-fly zone and called for the protection of civilians from Gaddafi's regime following an order for his forces to crush the revolt against his four-decade autocratic rule. NATO in its latest operation update released on Wednesday said air strikes in the vicinity of Tripoli had hit two tanks, two armoured vehicles, two surface-to-air missile launchers and two radar systems. In Geneva, the United Nations raised its aid funding appeal to $407.8 million from $310 million to help over two million people affected by the Libyan conflict. The sum would extend aid to September 2011, and would help 1.6 million people within Libya, as well as 500,000 others who have fled the country. Donors have so far provided about 43 percent of 2the required amount. In other developments, Libya freed four arrested journalists - two Americans, a Briton and a Spaniard - an AFP journalist witnessed as they arrived at the capital's Rixos Hotel. American James Foley of GlobalPost, an online news agency, and freelance writer Clare Morgana Gillis, as well as Spanish photographer Manu Brabo disappeared on April 4 while covering the conflict in Libya. They were released along with Briton Nigel Chandler, about whom no details were immediately available. On Tuesday, government spokesman Mussa Ibrahim said the two Americans, the Spaniard and a South African, Anton Hammerl, were to be released imminently. But there was no immediate information on the fate of Hammerl, who was also detained on April 4, even though the South African ambassador was at the hotel waiting for him. Ibrahim said his whereabouts were unknown.