Islamist premier Ali Larayedh on Tuesday accused Tunisia's main Salafist movement of liaising with Al-Qaeda and carrying out "terrorist" attacks that have rocked the country since the 2011 revolution, including two political assassinations.
Ansar al-Sharia "is responsible for the assassinations of (Chokri) Belaid and (Mohamed) Brahmi, as well as our martyrs in the police and the national army," Larayedh told a news conference, referring to the two politicians killed in separate attacks in February and July.
"We have decided to class Ansar al-Sharia as a 'terrorist organisation,'" he said, claiming that it was "liaising" with Al-Qaeda in the Islamic Maghreb (AQIM), the jihadist network's North African affiliate.
Larayedh's moderate Islamist party Ennahda, which heads the ruling coalition, has been heavily criticised for not doing enough to prevent waves of violence blamed on the extremists.
The prime minister vowed on Tuesday that Tunisia would fight against the group "whatever the sacrifices."
The assassinations of Belaid and Brahmi, within just six months of each other, both triggered crises in the North African country, with Belaid's murder in February bringing down the first Ennahda-led government, headed by Larayedh's predecessor Hamadi Jebali.
The killing of Brahmi, on July 25, has also sparked mass protests and calls for the immediate resignation of Larayedh's government and the formation of a non-partisan administration.
The authorities had already accused members of Ansar al-Sharia of involvement in the two killings, but they had not held the movement itself directly responsible.
Larayedh also said the extremist group was supporting armed jihadists holed up in the remote Mount Chaambi region, along the Algerian border, whom the Tunisian army has been hunting for months.
More than 10 soldiers have been killed since the army intensified its operations there at the end of July.
"This organisation is implicated in the terrorist operations in Tunisia," Larayedh said.
"It is responsible for a weapons storage network, it is responsible for planning assassinations, and attacks against security and army posts," he added.
The Tunisian premier said his claims were based on evidence and the "confessions of suspects."
"Everyone belonging to this organisation must assume complete responsibility for belonging to a terrorist organisation."
The government has been accused of failing to rein in Tunisia's radical Islamists, who were harshly repressed under the regime of ousted president Zine El Abidine Ben Ali.
Ennahda initially favoured dialogue with the Salafists.
But in recent months, following growing calls for action from opposition parties and civil society groups, it has taken a much tougher stand towards the extremist groups, and Ansar al-Sharia in particular.
Analyst Kevin Casey, with the Carnegie Endowment for International Peace Studies, said the ruling party's strategy of accommodation appeared to have "backfired."
"In spite of Ennahda's sincere attempts to open a dialogue with the Salafi movement, activism and violence steadily increased through 2012 and 2013," he noted in a report last week, adding that there was little indication of attacks decreasing since the government adopted its more confrontational approach.
Ansar al-Sharia's leader, Saif Allah Bin Hussein, a former Al-Qaeda fighter in Afghanistan known as Abu Iyadh, is accused of orchestrating a deadly attack on the US embassy in Tunisia last September and has been on the run from the police ever since.
His Salafist group, which was founded shortly after the popular uprising that toppled Ben Ali and claims some 40,000 supporters, has always denied any involvement in the violence that has intermittently rocked Tunisia since the revolution.
It insists its activities are focused on preaching and charitable work.
But in May Abu Iyadh threatened to wage war against the government, after the authorities banned the movement's annual congress and sent reinforcements to combat jihadists in the western border region.