Suspected jihadists killed 14 Tunisian soldiers near the Algerian border, officials said Thursday, in the worst such attack in the army's history as it presses a crackdown on radical Islamists.

The attack took place in the Mount Chaambi region on Wednesday evening as the soldiers were breaking their day-long Ramadan fast.

The defence ministry said 40 to 60 "terrorists", a term used to refer to Al-Qaeda-linked jihadists, opened fire on twin army posts with machine-guns and grenade launchers.

Fourteen soldiers died, 18 were wounded and another went missing, making it the heaviest toll inflicted on the army since Tunisia's independence in 1956, the ministry said.

"Most of the terrorists infiltrated from outside the Chaambi region, probably from outside the borders," said Defence Minister Ghazi Jeribi.

He said Tunisians, Algerians and unidentified "foreign mercenaries" were among the assailants. The army said a Tunisian jihadist was killed.

The ministry has previously insisted that militants the army has been hunting since late 2012 in the remote western border region are linked to Al-Qaeda in the Islamic Maghreb (AQIM).

The army said nine of the soldiers had burned to death.

"These attacks cost us 14 martyrs. Five were hit by bullets and nine were burned ... when the tent they were in caught fire," after it was hit by grenades, the army's head of ground operations, Souheil Chmengui, told a press conference.

"This is open war. The war of a country and a people against an evil," he said, pledging to combat terrorism "day and night".

The army's chief of staff, Mohamed Salah Hamdi, said it was unclear if the missing soldier had been killed, wounded or taken hostage.

President Moncef Marzouki declared three days of national mourning, while government spokesman Nidhal Ouerfelli condemned what he called a "heinous act".

The attacks came almost a year to the day after the bodies of eight Tunisian soldiers were found, some of them with their throats slit, after being an ambush in the western Kasserine region.

- Persistent jihadist threat -

Since the 2011 revolution that toppled Zine El Abidine Ben Ali, Tunisia has been rocked by violence blamed on hardline Islamists who were suppressed under the former dictator.

The country has made tangible progress this year in terms of stability, and elections are due to take place in just over three months, after political turmoil in 2013 triggered by the assassination of two opposition politicians.

But Interior Minister Lotfi Ben Jeddou said the aim of assailants was to undermine the political transition by forcing the October 26 legislative election and the November 23 presidential poll to be delayed.

In January, after a tense standoff between rival factions, parliament finally adopted a new constitution and the ruling coalition led by moderate Islamist party Ennahda resigned, allowing for the formation of an interim administration of independents.

But the jihadist threat has cast a shadow over any political gains.

Four Tunisian soldiers were killed by a land mine in the northwest Kef region during an anti-terror operation earlier this month, a day after four soldiers and two policemen were wounded by a roadside bomb in the same area.

Last month AQIM, the global terror network's North Africa branch, for the first time claimed responsibility for recent attacks in Tunisia, including an assault on the home of the interior minister that killed four security guards.

Officials insist the May 27 attack Ben Jeddou's home in the Kasserine region was a "revenge" attack in response to successes achieved by the security forces in their anti-terrorist campaign.

The authorities have designated Mount Chaambi and neighbouring mountain districts a closed military zone, and warned of the growing threat posed by "terrorist organisations" based there.

Since December 2012, Tunisia's security forces have been battling jihadists hiding in Mount Chaambi and Kef regions, which straddle the Algerian border, launching a series of major land and air operations.