Forces loyal to Libya's transitional authorities have taken the airport in the city of Sirte, the birthplace of fugitive leader Muammar Gaddafi.Fighters moved through the partially destroyed terminal buildings tearing down symbols of the Gaddafi regime.The whereabouts of the ousted leader remain unknown but several members of his family have fled Libya.His son Saadi is in Niger. Hours after an arrest warrant was issued, Niger's PM said he would not be extradited.International police agency Interpol's "red notice" for Saadi Gaddafi's arrest says he is wanted on allegations of misappropriating properties through force and armed intimidation when he headed the Libyan Football Federation".The agency said that as the commander of military units alleged to have been involved in violence against civilians during the uprising, he was also subject to a UN travel ban and assets freeze.Interpol said had last been seen in Niger and urged member states to help locate him and return him to Libya.But speaking in France, Niger's Prime Minister Brigi Rafini said Saadi Gaddafi was safe and "in the hands of the Niger government" in the capital Niamey."There's no question of him being extradited to Libya for the moment," Mr Rafini told the AFP news agency, saying he would not receive a fair trial if sent home.Interpol has already issued red notices for Col Gaddafi and another son, Saif al-Islam. Nato strikesGaddafi loyalists have been putting up stiff resistance in Sirte since the troops supporting the National Transitional Council (NTC) began their assault.Two weeks ago NTC forces took the airport, a short distance from the city centre, but were then driven back.The BBC's Jonathan Head says that this time they hope to hold it, despite facing continued rocket and gunfire from the other side of the runway.Fighters arriving from the west and east say they will then mount a joint attack on Gaddafi loyalists, hoping to push them back towards the sea and to squeeze them into ever smaller areas of the city centre, our correspondent adds.The airport is about 5km (3 miles) from central Sirte.Forces loyal to Libya's National Transitional Council (NTC) also recently seized Sirte's port.Heavy fighting was reported to be continuing there on Thursday, with the two sides shelling each others' positions and trading machine-gun fire.Nato planes have been carrying out air strikes in the area against military targets including ammunition storage facilities.Sirte and the city of Bani Walid are the last major areas under the control of Gaddafi loyalists, and both have seen heavy fighting in recent days.In Bani Walid, NTC forces appeared to be making little progress on Thursday."There is always incoming missile and artillery fire. We are returning fire with heavy weapons but we are not sending in infantry. We are waiting for reinforcements," NTC commander Walid Khaimej told AFP news agency."Nato is here but is not doing enough. They take out the rocket launchers firing at us, but they are immediately replaced."NTC officials have said they believe Col Gaddafi may be hiding in Libya's southern desert.They said that one of his sons, Mutassim, may be in Sirte, and another son, Saif al-Islam, in Bani Walid.Col Gaddafi's wife and three of his children fled to Algeria last month. A number of his entourage went further south, to Niger.Meanwhile, prosecutors in Tunisia said they had received a request for the extradition of former Libyan Prime Minister al-Baghdadi Ali al-Mahmoudi and were therefore keeping him in detention.Mr Mahmoudi had been sentenced to six months in prison last week on charges of entering the country illegally, but an appeals court overturned his conviction and ordered his release on Thursday.