(Adds details on security chief, background)
LONDON, Oct 21 (Reuters) - Muammar Gaddafi's son Saif al-Islam is fleeing south from Sirte towards Libya's border with Niger, a senior military commander of the interim National Transitional Council (NTC) said on Friday.
Abdul Majid Mlegta told Reuters that Islam was believed to travelling in a convoy of three armoured vehicles to try to escape NTC forces that overran Sirte on Thursday and killed his father, Libya's deposed former ruler.
"We are searching for him. The fighters in the region are on full alert," Mlegta said.
Gaddafi was captured alive in his hometown of Sirte on Thursday but died later while in the hands of fighters in circumstances that are still not clear. Islam is believed to have fled Sirte at about the same time.
Dozens of Gaddafi loyalists, including one of his sons Saadi, fled to Niger in September and are being sheltered in Niamey.
Niger has resisted appeals for them to be handed over to Libya's new rulers, saying Tripoli could send investigators if it wanted to question them.
Megta said that in recent days Gaddafi's security chief Abdullah al-Senussi, believed to be hiding in Niger, had been trying to organise some form of safe passage from Sirte to Niger for Gaddafi's entourage.
On Thursday Niger's foreign minister Mohammed Bazoum said he had been informed by Western countries that Senussi had fled across the border into the far north of Niger.
Senussi is wanted by the International Criminal Court for crimes against humanity.
The brother-in-law of the former Libyan leader, who was killed on Thursday, Senussi has been accused of ordering the murder and persecution of civilians throughout Libya during the collapse of Gaddafi's 42-year rule this year.
"It seems that he is in the extreme north of Niger. It is the Western countries which have informed us," Bazoum told Reuters by telephone from Niger.
International police agency Interpol on Thursday appealed for Islam to turn himself in, offering to guarantee his safe passage to The Hague to face charges of crimes against humanity.
Reporting by Samia Nakhoul; Editing by Myra MacDonald