* The women are on the Tunisian island of Djerba - source
* Daughter Aisha defended her father's role
* Reason for trip unclear
(Adds background on Aisha)
By Tarek Amara
TUNIS, May 18 (Reuters) - The wife and daughter of Libyan leader Muammar Gaddafi crossed over the border into Tunisia several days ago, a Tunisian security source said on Wednesday.
Gaddafi's wife Safia and his daughter Aisha came to Tunisia with a Libyan delegation on May 14 and are on the island of Djerba in the south, the source told Reuters.
"It was expected that they would leave yesterday but they are still at Djerba," the source added.
It did not appear that the two women had been travelling with Shokri Ghanem, Libya's top oil official, who is believed to have also crossed into Tunisia and appears to have defected.
Libyan officials in Tripoli were not immediately available for comment.
Since the revolt began in February against Gaddafi's rule, Aisha Gaddafi has made several public appearances backing her father and attacking the rebels and Western powers trying to overthrow him.
A month ago she appeared at her father's Bab al-Aziziyah compound in Tripoli and addressed cheering crowds of supporters in an event broadcast live on Libyan state television.
"Talk about Gaddafi stepping down is an insult to all Libyans because Gaddafi is not in Libya, but in the hearts of all Libyans," she said.
A lawyer by training, Gaddafi's daughter runs a charitable foundation and in 2004 joined a team of lawyers defending former Iraqi leader Saddam Hussein. (Additional reporting by Christian Lowe, Writing by Sylvia Westall; Editing by Maria Golovnina)