Tunisian special forces are seen during clashes Monday with militants in Ben Guerdane, near the Libyan border. (Fathi Nasri/AFP/Getty Images)

Gunmen apparently crossing from Libya attacked a Tunisian border garrison on Monday, officials said, raising worries about Islamic State fighters and other militants gaining greater footholds in North Africa. More than 50 people, including 35 of the attackers, were left dead in the clashes.

There was no immediate claim of responsibility, but militants thought to be linked to the Islamic State carried out two major attacks last year in Tunisia targeting the country’s important tourism industry.

Tunisia’s president, Beji Caid Essebsi, called Monday’s strike “an unprecedented attack, planned and organized.”

“The majority of Tunisians are now in a state of war against barbarism,” he said from the capital, Tunis.

The assault followed skirmishes in the region last week between Tunisian forces and well-armed militiamen also thought to be based in Libya, where the Islamic State has expanded its presence amid political chaos that has divided the country into two rival zones.

Tunisian forces have been on elevated alert for possible cross-border incursions since a U.S. airstrike Feb. 19 targeting a suspected Islamic State training camp outside the Libyan town of Sabratha, near the Tunisian frontier.

Tunisia’s government tallied at least 53 deaths: 35 attackers, 11 members of Tunisia’s security forces and seven civilians, including a 12-year-old girl.

Tunisian officials closed border crossings hours after the predawn attack on the compound in Ben Guerdane, used by police and soldiers. In Paris, France’s Foreign Ministry identified the gunmen as “coming from Libyan territory.”

In an interview with Tunisian radio station Shems FM, a resident of Ben Guerdane said that moments before the attack, an unfamiliar voice took over the local mosque’s call to prayer. “The call to prayer was not a normal one today. There was a different voice and then the shooting began,” the unidentified resident said.

The Islamic State is among a host of armed factions in Libya, but its attempts to expand its hold have drawn increasing alarm from the West and neighbors such as Tunisia. Last month, President Obama urged greater efforts to keep the Islamic State from “digging in” across Libya.

In the U.S. airstrike, a key target was a Tunisian militant, Noureddine Chouchane, who helped train fighters for last year’s attacks in Tunisia, according to Western officials and others. U.S. officials said at the time that Chouchane was probably killed in the air raids.

Islamic State fighters are estimated to number between 2,000 and 5,000 in several cells across Libya. Some of those are believed to be Libyans who left other militant groups, while others are foreigners, including many Tunisians.

In March 2015, gunmen killed 22 people at the National Bardo Museum, a popular destination for visitors in Tunis. Three months later, an attack on a beach resort in Sousse left 38 people dead, many of them British nationals.

Murphy reported from Washington. Sudarsan Raghavan in Cairo contributed to this report.

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