Twelve people were killed in Tuesday's explosion in Tunis, according to the Interior Ministry. A suspected suicide bomber also died.
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
, 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.
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."
'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 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."
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
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.
The attacks strike at the heart of Tunisia's beleaguered tourism industry
-- already struggling to rebound after the upheavals of the Arab Spring. Many Tunisians' jobs rely on tourism, which accounts for some 15% of the country's annual gross domestic product, according to a report from the World Travel and Tourism Council