The state news agency in Egypt has confirmed that Libyan interior minister Nasser al-Mabrouk Abdullah is in Cairo with members of his family.
It is not clear if he has defected or is on holiday.
The BBC reports he arrived on a special plane from Tunisia and told Egyptian officials that he was ''on a tourist visit''.
This has led to speculation of a possible defection from Colonel Muammar Gaddafi's inner circle.
Meanwhile, rebel fighters are advancing on Tripoli. After a weekend of heavy fighting, the rebels say they have taken control of Sorman, west of Tripoli, and Gharyan to the south.
They are reportedly advancing against government forces in the key oil town of Zawiya, 50km west of Tripoli.
Although the capital is not under immediate threat, the BBC reports the rebel forces are now in their strongest position since the uprising began in February.
Colonel Gaddafi has ruled for 41 years.