Tunis, Tunisia (CNN) ISIS has claimed responsibility for bombing a bus carrying members of Tunisia's presidential guard, the latest deadly attack to roil a country once hailed as one of the few success stories of the Arab Spring .

Twelve people were killed in Tuesday's explosion in Tunis, according to the Interior Ministry. A suspected suicide bomber also died.

A Syrian family waits after arriving on the Greek island of Lesbos along with other migrants and refugees, on November 17, 2015. Ten of the 11 countries most affected by terrorism also have the highest rates of refugees.

A Syrian family waits after arriving on the Greek island of Lesbos along with other migrants and refugees, on November 17, 2015. Ten of the 11 countries most affected by terrorism also have the highest rates of refugees.

An Iraqi Kurdish Peshmerga fighter prepares an ammunition belt as he guards a position at the frontline of fighting against ISIS militants near the northern Iraqi town of Sinjar.

An Iraqi Kurdish Peshmerga fighter prepares an ammunition belt as he guards a position at the frontline of fighting against ISIS militants near the northern Iraqi town of Sinjar.

Iraqi counter-terrorism forces patrol a street in Tikrit in April 2015, a day after the country's prime minister declared victory in the battle to retake the city from ISIS.

Iraqi counter-terrorism forces patrol a street in Tikrit in April 2015, a day after the country's prime minister declared victory in the battle to retake the city from ISIS.

A member of the Iraqi security forces fires ammunition in Baghdad in 2014 during a funeral procession of an Iraqi politician. The deadliest city in the world for terrorism is Baghdad. There were 2,454 deaths in Baghdad in 2014, with a death rate from terrorism of 43 per 100,000 people.

A member of the Iraqi security forces fires ammunition in Baghdad in 2014 during a funeral procession of an Iraqi politician. The deadliest city in the world for terrorism is Baghdad. There were 2,454 deaths in Baghdad in 2014, with a death rate from terrorism of 43 per 100,000 people.

In this image taken from social media, an ISIS fighter holds the group's flag as he stands on a tank, purportedly captured when they took over the town of Qaryatain, Syria. ISIS, along with Boko Haram, is one of two groups accounting for 51% of claimed terrorism deaths in 2014.

In this image taken from social media, an ISIS fighter holds the group's flag as he stands on a tank, purportedly captured when they took over the town of Qaryatain, Syria. ISIS, along with Boko Haram, is one of two groups accounting for 51% of claimed terrorism deaths in 2014.

A Nigerian policeman inspects the site of a suicide attack by Boko Haram at a busy cattle market in the northeastern Nigerian city of Maiduguri in June, 2015. Boko Haram overtook ISIS as the world's deadliest terror group last year, according to the Global Terrorism Index, while Nigeria had the biggest year-on-year increase in terrorism, with deaths up more than 300%.

A Nigerian policeman inspects the site of a suicide attack by Boko Haram at a busy cattle market in the northeastern Nigerian city of Maiduguri in June, 2015. Boko Haram overtook ISIS as the world's deadliest terror group last year, according to the Global Terrorism Index, while Nigeria had the biggest year-on-year increase in terrorism, with deaths up more than 300%.

The blast hit when the vehicle was parked near a main artery in the capital where guard members are typically picked up and dropped off, according to the official. Authorities are investigating the event as a terrorist attack.

ISIS, the Islamist extremist group that has taken over vast swaths of Syria and Iraq , claimed responsibility for the attack in a statement released Wednesday.

The statement said a suicide bomber blew up his explosive vest on the bus.

It contained a photo of the alleged suicide bomber, along with the message: "Tyrants of Tunis must know that they will not be safe, God willing, and we won't stop until the law of God rules Tunis."

The Tunisian government has not commented on the ISIS statement. CNN cannot authenticate the ISIS claim.

International condemnation

A spokesman for U.N. Secretary-General Ban Ki-moon said in a statement that he sent condolences to Tunisia and the victims' families.

"The Secretary-General reiterates that the United Nations will continue to stand with the people of Tunisia as they confront the scourge of terrorism and continue to consolidate and strengthen their democracy," the statement said.

The United States condemned the attack, saying it was prepared to assist Tunisian authorities in their investigation.

"Terrorists have sought to use fear and violence to undermine the important gains the Tunisian people have made in pursuit of a democratic, stable, and prosperous country," National Security Council spokesman Ned Price said in a statement.

British Foreign Secretary Philip Hammond also tweeted his support, writing that the UK stood with Tunisia and would "support Tunisian security forces confronting terrorism."

Condemn appalling #TunisiaAttack. UK stands with #Tunisia and will support Tunisian security forces confronting terrorism — Philip Hammond (@PHammondMP) November 24, 2015

'Jasmine Revolution' sours

Tunisia -- the birthplace of the Arab Spring -- has been considered a rare success story of a regional uprising that led to a brutal civil war in Syria , a failing state in Libya , a repressive counterrevolution in Egypt and other bleak outcomes.

Last month, the Nobel Peace Prize Committee surprised everyone and awarded the prize award to the Tunisian National Dialogue Quartet for its "decisive contribution to the building of a pluralistic democracy in the country in the wake of the Jasmine Revolution of 2011."

But the past year has seen a surge in Islamist violence that has raised fears for the future of this North African nation of 11 million people.

In March, 22 people, most of them tourists, were killed in an attack on the Bardo Museum in Tunis.

In June, 38 people, most of them British, where killed when a jihadist gunman went on a rampage at a seaside hotel at a resort in Sousse.

In the wake of the Sousse attack, Tunisian President Beji Caid Essebsi warned that another such massacre would "cause the country to collapse."

Photos: Terrorist attack in Tunisia Photos: Terrorist attack in Tunisia People lay flowers on the beach in front of Hotel Riu Imperial Marhaba, in the coastal city of Sousse, Tunisia, on Saturday, June 27. Gunmen killed at least 38 people at site , the same day terrorists lashed out brutally in France and bombed a mosque in Kuwait. Hide Caption 1 of 14 Photos: Terrorist attack in Tunisia Tunisian police patrol the beach in front of the Riu Imperial Marhaba Hotel on June 27. Hide Caption 2 of 14 Photos: Terrorist attack in Tunisia A Tunisian forensics expert inspects the beach on June 27. Hide Caption 3 of 14 Photos: Terrorist attack in Tunisia A young woman lays flowers at the scene of a shooting on June 27. Hide Caption 4 of 14 Photos: Terrorist attack in Tunisia Tourists leave Tunisia at the Enfidha International airport a day after the attack. Hide Caption 5 of 14 Photos: Terrorist attack in Tunisia Tunisian security forces stand in front of Hotel Riu Imperial Marhaba on Friday, June 26. Hide Caption 6 of 14 Photos: Terrorist attack in Tunisia Tunisian security forces evacuate the hotel. At least one gunman was reportedly killed, according to the state-run TAP news agency. Hide Caption 7 of 14 Photos: Terrorist attack in Tunisia On its website, Hotel Riu Imperial Marhaba is described as an all-inclusive hotel with views of Port El Kantaoui. Hide Caption 8 of 14 Photos: Terrorist attack in Tunisia Tunisian President Beji Caid Essebsi, third from right, arrives at the resort with Prime Minister Habib Essid, third from left, and Interior Minister Mohamed Najem Gharsalli, far left. Hide Caption 9 of 14 Photos: Terrorist attack in Tunisia Hide Caption 10 of 14 Photos: Terrorist attack in Tunisia A medic stands next to the body of a tourist. Hide Caption 11 of 14 Photos: Terrorist attack in Tunisia Dead bodies are seen on the beach. Hide Caption 12 of 14 Photos: Terrorist attack in Tunisia A Tunisian security member stands next to a swimming pool at the hotel. Hide Caption 13 of 14 Photos: Terrorist attack in Tunisia Tunisian medics stand near a tourist's body. Hide Caption 14 of 14

ISIS also claimed responsibility for both those attacks.

Tunisia has contributed the most foreign fighters to join Sunni militant groups such as ISIS in Syria and Iraq, according to a report published by the International Center for the Study of Radicalization and Political Violence this year.

The organization estimated that 1,500 to 3,000 Tunisians were waging jihad in Syria and Iraq, with Saudi Arabia second with 1,500 to 2,500. By contrast, France had contributed the most foreign fighters of any Western country, with 1,200.