One of the deadliest clashes seen so far between Tunisian forces and extremist attackers left at least 45 people dead Monday near Tunisia's border with Libya, the government said.
The fighting in the border town of Ben Guerdane in eastern Tunisia comes amid increasing concern that violent extremism in Libya could destabilize the region.
"This is an unprecedented attack, planned and organized, and whose goal was probably to take control of this area and to announce a new emirate," said Tunisian President Beji Caid Essebsi.
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The government closed its two border crossings with Libya following the attack that left 35 extremists, seven civilians and 11 members of the security forces dead, the Tunisian interior and defence ministries said in a statement.
A 12-year-old girl was among those who were killed.
Libya's chaos, five years after the uprising that led to the ouster and killing of longtime autocrat Moammar Gadhafi in 2011, has allowed the Islamic State group to take control of several cities. The divided country is ruled by two parliaments: an internationally recognized body based in the eastern city of Tobruk and a rival government, backed by Islamist-allied militias, that controls the capital, Tripoli.
Tunisia's fledgling democratic government is especially worried about the presence of ISIS in Libya after dozens of tourists were killed in extremist attacks in Tunisia last year.
At dawn Monday, gunmen targeted a police station and military facilities in Ben Guerdane, Interior Ministry spokesman Yasser Mosbah told The Associated Press. A night curfew has been ordered in Ben Guerdane until further notice.
No group has claimed responsibility for the attacks, but members of the jihadist forum community commenting on two ISIS-affiliated websites said Islamic State group militants were engaged in the fighting.
Tunisia's prime minister said in a televised speech Monday night the attack was an effort by the Islamic State group to establish a stronghold in the region.
Habib Essid said Tunisian security forces had achieved "positive results" in response to the attack, noting that Tunisia estimates that 35 attackers are among the 53 dead.
Essebsi said extremists now pose a security threat to the entire country.
Speaking from the capital Tunis, Essebsi said Monday: "This is an unprecedented attack, planned and organized. Its goal was probably to take control of this area and to announce a new emirate. The majority of Tunisians are now in a state of war against barbarism."
France's foreign ministry condemned the attacks and identified the gunmen as "terrorists coming from Libyan territory."
"This attack is just reinforcing the urgent need for a political solution in Libya," the ministry said in a statement, adding that Tunisia was targeted because of its "exemplary democratic transition."
Hospital official Abdelkrim Sakroud said on state radio that three corpses had been brought in, including that of the 12-year-old girl.
The Tunisian military sent reinforcements and helicopters to the area around Ben Guerdane and authorities were hunting several attackers still at large. Officials urged residents to stay indoors.