TRIPOLI, Libya — Reports that a top official in the government of Moammar Gaddafi has defected gained strength here on Tuesday, as a spokesman for the Libyan government said the regime has been unable to make contact with the official. Shukri Ghanem, Libya’s oil minister and a former prime minister, left Libya on Monday to go to Tunisia on “official business,” said Moussa Ibrahim, spokesman for the Libyan government. But news agencies reported Tuesday that Ghanem had defected, citing an anonymous Tunisian security official.