"This offer is no longer valid," Moustafa Abdel-Jalil told reports in the rebel-held city of Benghazi.

"The offer was made about a month ago through UN envoy Abdel Elah al-Khatib, with a two-week deadline," he added, noting that the two weeks had passed.

Jalil, the former justice minister, took the reins of the rebel movements after resigning from Gaddafi's government in February over what he saw as excessive use of force against demonstrators calling for the leader's ouster.

Rebels have been battling Gaddafi's forces for months in an attempt to bring about an end to the Libyan leader's 42-year rule.

Gaddafi has been under international pressure to step down, and the International Criminal Court at the Hague issued a warrant for his arrest as well as for the arrest of his son, Saif al-Islam and his brother-in-law Abdullah al-Sanussi.