Tunisian forces killed 28 jihadists who attacked police and army posts near the Libyan border on Monday in a new spillover of violence that also saw four civilians killed.

Authorities ordered a nighttime curfew as troops deployed across the border town of Ben Guerdane to hunt for others involved in the coordinated pre-dawn attack.

It was the second deadly clash in the border area in less than a week as Tunisia battles to prevent the large number of its nationals who have joined the Islamic State (IS) jihadist group in Libya from returning to carry out attacks at home.

The jihadists have taken advantage of a power vacuum since the NATO-backed overthrow of longtime dictator Moamer Kadhafi in 2011 to set up bases in several areas of Libya, including the Sabratha area between Tripoli and the Tunisian border.

The government said that an army barracks and police and National Guard posts in Ben Guerdane came under simultaneous attack.

A soldier was also killed in the fighting and six wounded militants were captured, the defence ministry said.

Hospital official Abdelkrim Chafroud said a 12-year-old was among the dead civilians, and two security agents were also killed.

An AFP correspondent reported that schools and offices in Ben Guerdane were closed and troops had taken up position on rooftops across the town.

Residents were being urged to stay indoors even before the 7 pm (1800 GMT) start of the nighttime curfew.

Authorities closed the border crossings with Libya. They also closed the main road north to the rest of Tunisia, the correspondent said.

Prime Minister Habib Essid ordered the defence and interior ministers to head to Ben Guerdane to oversee operations against the jihadists.

Last Wednesday, troops killed five militants in a firefight outside the town in which a civilian was also killed and a commander wounded.

Troops have been on alert in the border area following reports that militants had been slipping across since a US air strike on an IS training camp in Libya on February 18 killed dozens of Tunisian militants.

At least four of the five militants killed in last week's firefight were Tunisians who had entered from Libya in a bid to carry out attacks in their homeland, the interior ministry said.

Deadly attacks by IS on foreign holidaymakers last year, which dealt a devastating blow to Tunisia's tourism industry, are believed to have been planned from Libya.

Tunisia has built a 200-kilometre (125-mile) barrier that stretches about half the length of its border with Libya in an attempt to stop militants infiltrating.

JIHADIST PRESENCE GROWING

February's US strike on the IS training camp outside Sabratha targeted the suspected mastermind of two of last year's attacks, Noureddine Chouchane.

Washington has said Chouchane was likely among the dozens of militants killed, and that the strike probably averted a mass shooting or similar attack in Tunisia.

Western governments have been increasingly alarmed by the growing IS presence in Libya just 300 kilometres (185 miles) across the Mediterranean from Europe and have made contingency plans for intensified military action.

Rival administrations which have vied for power since mid-2014 are being urged to sign up to a UN-brokered national unity government to facilitate the fight against the jihadists.

Handfuls of US, British and French special forces have already been reported in Libya.

A contingent of around 50 Italians is about to join them, Il Corriere della Sera reported last Thursday, citing a classified order signed by Prime Minister Matteo Renzi last month.

Britain announced last week that it was sending a team of around 20 soldiers to Tunisia to train troops patrolling the border with Libya.

Thirty Britons were among 38 foreign holidaymakers killed in a gun and grenade attack on a beach resort near the Tunisian city of Sousse last June.

And last March, jihadist gunmen killed 21 tourists and a policeman at the Bardo Museum in Tunis.

According to a UN working group on the use of mercenaries, more than 5,000 Tunisians have travelled abroad to join jihadist groups, many of them in Libya.