Three leaders of an Al-Qaida-linked jihadist group blamed for a spate of violence in Tunisia were among five "terrorists" killed last week in a security raid, the interior minister has said.

Najem Gharsalli said the three jihadists were "heavyweights" from the Okba Ibn Nafaa Brigades, Tunisia's main extremist group which had been blamed for the March attack on the Bardo National Museum.

The jihadist Islamic State group on Tuesday claimed responsibility for an attack in central Tunisia that killed three policemen and wounded 12.

In a statement posted on Twitter, IS said the attack in the Sidi Bouzid region was carried out by "two soldiers of the caliphate who attacked two military sites with small arms."

Dozens of Tunisians held in Libya by a militia forming part of an alliance ruling Tripoli have been freed, Tunisia's foreign ministry announced Saturday.

"All the Tunisians being held in Libya have been freed. The final group was released today," the ministry said in a statement, without giving numbers.

Tunisia has arrested a second Moroccan suspect over a deadly attack in March on the Bardo museum in Tunis, detaining him at the border with Libya, the interior ministry said Thursday.

Noureddine al-Naibi was arrested on Sunday at the Ras Jedir border post on suspicion of indirect involvement in the attack, ministry spokesman Mohamed Ali Aroui told AFP.

It's another sun-drenched day but the deckchairs of a four-star hotel on the island of Djerba stand empty, in a sign of the "catastrophic" summer on the horizon for Tunisian tourism.

"It's dead. It's all over for this year," was the blunt verdict from Adel Tarres, manager of the hotel on the holiday island off the south coast of Tunisia that has in past years been a magnet for tourists from Italy across the Mediterranean.

Tunisia said Monday that it was negotiating for the release of 172 nationals being held by a Libyan militia as bargaining chips for one of its commanders detained in Tunis.

The foreign ministry's Arab and African affairs chief Touhami Abdouli said the Tunisians had been detained in western Libya by a militia that is part of the Libya Dawn alliance which controls the capital Tripoli and third city Misrata.

Tunisia said Sunday security measures have already been taken to protect Jewish pilgrims at a religious festival next week on the island of Djerba, after Israel warned of "concrete threats."

Prime Minister Benjamin Netanyahu said on Saturday that Israel had learned of "concrete threats" of terror attacks against Jewish or Israeli targets in the North African country, prompting a quick denial from Tunis.

Protesters took to the streets of Tunis Friday demanding to know the truth about the fate of two journalists whose execution-style killing in Libya was claimed by the Islamic State group.

"Give us back Sofiene; give us back Nadhir," the crowd of almost 200 demonstrators chanted as they brandished pictures of Sofiene Chourabi and Nadhir Ktari.

Tunisia's national museum was forced to delay its planned reopening on Tuesday, officials said, as security was boosted around the site of last week's jihadist massacre that killed 21 people.

Hundreds gathered outside the National Bardo Museum in central Tunis, many carrying placards encouraging foreigners to visit Tunisia which relies heavily on tourism income.

Tunisian President Beji Caid Essebsi said security "failures" had helped facilitate the deadly attack on the country's national museum, according to an interview with French media published Saturday.

"There were failures" which meant that "the police and intelligence were not systematic enough to ensure the safety of the museum", Essebsi told the weekly Paris Match.