An aide to Libya's Prime Minister Ali Zeidan has been freed after being kidnapped nine days ago, officials say.Mohamed al-Ghattous, was said to be with his family in Misrata city following his release.An official statement did not give details of who captured him or how his release was secured.Libya has been hit by instability since the overthrow and killing of ex-leader Muammar Gaddafi in October 2011.The government has been battling to disarm and disband the militias that had been formed to fight Col Gaddafi's forces.Torture bannedIn a statement, Mr Zeidan's office confirmed that Mr al-Ghattous had been released."We thank everyone for all the efforts exercised by the various sides and who made contacts and tried helping to secure his release and return," it said.Mohamed al-Ghattous is believed to have been seized on 1 April on the outskirts of the capital, Tripoli, as he was driving from his hometown of Misrata, about 210km (130 miles) away.The BBC's Rana Jawad reports from Tripoli that Libyans suspect some militias are involved in kidnappings, although details of such cases often remain vague.Militias also run Libya's prisons and illegal detention centres, she says.To end the culture of impunity, the National Congress has passed a law criminalising kidnappings, torture and illegal detentions, our reporter adds.The law stipulates that anyone convicted of kidnapping, illegally detaining, or jailing people by force or under false pretences face a prison sentence of no less than seven years.Torture will be penalised by five to 10 years in prison or a lifetime sentence if it results in death, the law statesCol Gaddafi's son Saif al-Islam, who was seen as a reformer during his father's rule, has been held by militiamen in the western town of Zintan since his capture at the end of 2011.Last year, his Australian lawyer Melinda Taylor was held for three weeks by militias after she visited him in captivity.Saif al-Islam appeared in court in January charged with trading information threatening national security.The trial was postponed until May as there was no lawyer to represent him.