Tunisia has designated the hardline Salafist Ansar al-Sharia movement a "terrorist group", blaming it for the killing of two secular politicians.
PM Ali Larayedh said he had proof it was behind the killings of Chokri Belaid and Mohamed Brahmi, which plunged Tunisia into political turmoil.
"Anyone belonging to it must face judicial consequences," he added.
The group, which emerged after the 2011 revolution, seeks the implementation of Islamic Sharia law across Tunisia.
Mr Larayedh also said the group was supporting an armed jihadist cell which the Tunisian army has been hunting for months in the remote Mount Chaambi region along the Algerian border.
The Tunisian army launched an offensive in the region last month after eight soldiers were ambushed and killed by gunmen suspected of links to al-Qaeda.
"We have discovered proof that the Ansar group is responsible for the assassinations of Chokri Belaid and Mohamed Brahmi and the attacks at Mount Chaambi," Prime Minister Larayedh told reporters on Tuesday.
He said Ansar al-Sharia was "liaising with AQIM (al-Qaeda in the Islamic Maghreb)", while pledging that Tunisia would fight against the group "whatever the sacrifices".
"This organisation is implicated in the terrorist operations in Tunisia," Mr 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.
Ansar leader Seif Allah Ibn Hussein, also known as Abu Iyadh, is currently in hiding after an arrest warrant was issued for allegedly inciting an attack on the US embassy in Tunis in September 2012, which killed four people.
Members of Ansar al-Sharia clashed with security forces in May after the authorities banned the group's annual congress for "inciting violence against state institutions".
The assassinations of Mr Belaid and Mr Brahmi within six months of each other plunged Tunisia into a political crisis.
The moderate Islamist Ennahda party - elected after the overthrow of President Zine el-Abidine Ben Ali in January 2011 - stands accused by its opponents of failing to rein in radical Islamists in the country.
Since coming to power, it has faced growing unrest - particularly among the youth - over a faltering economy and a rising radical Islamist movement.
The Ennahda-led coalition faced mass street protests after the 25 July killing of MP Mohamed Brahmi, the leader of the small left-wing Popular Movement party.
In February, the government led by Islamist Hamadi Jebali was brought down after prominent secular opposition leader Chokri Belaid was also assassinated.
Police later said the two men were killed by the same gun.