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Hundreds of Islamic State group [IS] militants have "disappeared" from its last bastion in Libya and could be regrouping inside the war-torn country and in Europe to carry out attacks.

Forces loyal to Libya's unity government announced its full control of Sirte on Monday, in a major blow to the extremists, and that dozens of IS fighters had surrendered.

"Many IS militants have mysteriously gone missing and we still have no answers," °®Âþµº Libya correspondent, Abdullah al-Sharif, said.

"Libyan estimates have put the number the number of militants in the city at 1,500 at best, while Western reports have said there were 5,000 to 6,000.

He added that escaped militants have likely found refuge in the country's lawless central and southern areas and will regroup to launch attacks at any time.

Extremists already have bases in Libya's vast desert regions and are working to build ties with local tribes.

"Most observers think they have relocated to the border region with Chad and Sudan or Algeria and Niger. This could eventually lead to a new area of internal conflict with tribal and ethnic factors," Sharif said.

The battle for Sirte cost the lives of around 700 loyalist troops and 1,000 IS fighters, a Bunyan al-Marsous media officer and medics have told °®Âþµº, leaving around 500 militants unaccounted for, according to Libyan estimates.

The Bunyan al-Marsous operation room has previously said it was likely hundreds of militants escaped the city around July before it was put fully under siege.

In August, sources told °®Âþµº that warplanes bombed an IS convoy south of Sirte as militants fled to take cover in the rugged desert terrain.

An EU report warned on Wednesday that foreign IS fighters in Libya could use their nationality or family connections to return to Europe and some may have orders to attack.

"Those in the majority that will drift back, and those who will be sent back on specific missions, which are of most concern," the report warned.

On Tuesday, loyalist forces said their soldiers were "chasing the last extremists hiding in fewer than 10 houses" in the seafront district of al-Giza al-Bahriya, the last to fall in the almost seven-month-long battle.