Libyan Oil Minister Shukri Ghanem has left the country, amid reports that he has defected.
Tunisian officials say Mr Ghanem crossed into Tunisia by road, before going to the island of Djerba, the BBC reports.
A spokesperson for the Libyan rebels says the minister has defected and is on his way to a European country.
The Libyan government said he had been on official business in Tunisia, but that Tripoli had lost touch with him.
Mr Ghanem, a former prime minister, is also the head of Libya's National Oil Corporation.
A number of senior officials have abandoned the government of the Libyan leader Muammar Gaddafi since the start of the uprising against him in February.
They include Foreign Minister Moussa Koussa, Interior Minister Abdul Fattah Younis, Justice Minister Mustafa Abdel Jalil, and a number of ambassadors.