Tunisia said Wednesday it is closing its border with Libya, a hotbed of extremist unrest, a day after a deadly suicide bombing claimed by the ISIL group.

Tunisia said Wednesday it is closing its border with Libya, a hotbed of extremist unrest, a day after a deadly suicide bombing claimed by the ISIL group.

No reason was given, but the interior ministry said earlier the explosive used in the attack that killed 12 presidential guards was the same used to make explosive belts illegally brought from Libya and seized last year.

The National Security Council, headed by President Beji Caid Essebsi, decided to close the frontier from midnight with "reinforced surveillance of maritime borders and in airports", a statement said.

It also decided to "step up operations to block (Internet) sites linked to terrorism".

And authorities would "take urgent measures regarding people returning from hotbeds of conflict, in line with the antiterrorist law," the statement added, without elaborating.

Earlier, the transport ministry said security would be reinforced at ports and only passengers would be allowed to enter Tunis's international airport.

Tunisia's President Beji Caid Essebsi had declared a nationwide state of emergency and a curfew in the capital after a bomb attack on a presidential guard bus killed at least 12 people on Tuesday.

A security source at the site said "most of the agents who were on the bus are dead" after the attack in Tunis, which has become a target of extremist violence since the 2011 revolution.

No group immediately claimed responsibility for the bombing, which a ministry official said also wounded 20 people when it went off on Mohamed V Avenue, just as this year's 26th Carthage Film Festival was in full swing.

Essebsi, who cancelled a trip to Switzerland for Wednesday, declared a state of emergency throughout the country and a curfew in the capital.

"As a result of this painful event, this great tragedy... I proclaim a state of emergency for 30 days under the terms of law and a curfew in greater Tunis from 9:00 pm (2000 GMT) until 5:00 am tomorrow," he said in brief televised address.

Presidential spokesman Moez Sinaoui told AFP the curfew would stay in place until further notice.

Prime Minister Habib Essid and Interior Minister Najem Gharsalli visited the scene of the blast.

The United States condemned the attack and offered to help the Tunisian authorities with their investigation.

US Secretary of State John Kerry "was proud to stand with Tunisian leaders earlier this month in Tunis and reaffirm our countries' extensive economic, governance, and security cooperation", a spokesman for his department said.

Tunisia has been plagued by extrimist violence since the 2011 overthrow of longtime dictator Zine El Abidine Ben Ali, and dozens of members of the security forces have also been killed.

Two attacks this year claimed by the Takfiri group, ISIL (so-called Islamic State in Iraq and Levant) targeted foreigners -- at the National Bardo Museum in March, killing 21 tourists and a policeman, and at a resort hotel in Sousse in June, killing 38 tourists.