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.

Dozens of Tunisians who had been detained in Libya by a militia were set free in the capital Tripoli on Thursday, officials said.

Mohamed Abdelsalam al-Kuwiri, who heads a unit in the Tripoli-based government that combats illegal migration, said his officers helped secure the release of 42 Tunisians.

U.S. President Barack Obama offered Tunisia closer security ties Thursday, in a bid to ensure jihadists do not extinguish the brightest democratic light to emerge from the Arab Spring.

Hosting President Beji Caid Essebsi at the White House, Obama declared Tunisia a "non-NATO ally," which would allow for enhanced military cooperation and the transfer of advanced weapons. 

Italy said Wednesday it had arrested a Moroccan illegal immigrant suspected of involvement in a deadly attack on a Tunis museum, fueling a row over the threat of jihadists arriving in Europe by boat.

Authorities in Tunisia have arrested nearly two dozen suspects in connection with the March attack in which 21 tourists died, but Abdel Majid Touil, 22, was thought to be the first to be detained abroad.

Human Rights Watch called Tuesday for a "transparent and credible" investigation by Tunisia into the suspicious death of a prisoner who had complained of police torture.

Abdelmajid Ejday died on May 13 in the National Guard headquarters in Sidi Bouzid, the birthplace of Tunisia's 2011 uprising against longtime strongman Zine El Abidine Ben Ali, HRW said in a statement.

Tunisian President Beji Caid Essebsi begins a two-day official visit to the United States Wednesday, seeking more military aid to counter the jihadist threat and the chaos in neighboring Libya.

He will meet U.S. President Barack Obama on Thursday after talks with Secretary of State John Kerry the day before.

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.

Four jihadists were killed Thursday in clashes with the Tunisian army in a mountainous region near the border with Algeria, the defense ministry said.

A statement said troops clashed with jihadists at dawn on Mount Sammama near Mount Chaambi, where the army has been battling militants since 2012.

The United States announced on Thursday the delivery of 52 Humvee military vehicles and a patrol boat to help Tunisia counter jihadist violence and ward off the insecurity of neighbouring Libya.

The 52 HMMWVs aim "to improve force mobility" of the Tunisian army, the U.S. embassy in Tunis said.

Rights groups including Amnesty International on Wednesday criticized a draft security law proposed by the Tunisian government to confront jihadists, saying it was "incompatible with international standards."

The bill, which is to be debated in parliament at an unspecified date, lays down stiff jail sentences for divulging state secrets or "denigrating" the army or police force.