NIAMEY/BEIJING (Reuters) - The prosecutor for the International Criminal Court (ICC) said on Sunday he has "substantial evidence" that Saif al-Islam Gaddafi, now on the run, had helped hire mercenaries to attack Libyan civilians protesting against his father's rule. Saif al-Islam may be heading for Niger, which could upset Libya's new rulers and its own pro-Gaddafi Tuareg nomads if it hands him over to the ICC in line with its treaty obligations. "We have a witness who explained how Saif was involved with the planning of the attacks against civilians, including in particular the hiring of core mercenaries from different countries and the transport of them," ICC prosecutor Luis Moreno-Ocampo told Reuters during a visit to Beijing.