A French military jet believed to be carrying ailing Palestinian leader Yasser Arafat landed Friday afternoon at an airfield outside Paris, witnesses said. Arafat was being brought to France for urgent treatment of a serious but undisclosed illness. He has been sick for two weeks, and blood tests revealed he has a low platelet count. He was to be taken immediately to the Hospital d'Instruction des Armees de Percy in southwest of Paris. The facility is a military hospital with a major trauma center but also specializes in the treatment of blood disorders, said Christian Estripeau, head of communications for military health services.