The Tunisian Government has declared a 30-day state of emergency after a bus exploded in Tunis, the Tunisian capital, on Tuesday.
A statement by the Tunisian Interior Ministry revealed that at least 12 people were killed and 17 wounded in the incident.
A curfew, which finished this morning, was also imposed in the Tunis region.
The country's security forces will exercise more power during the enforced emergency period.
In a statement, the US Department of State said: "The US strongly condemns today's terrorist attack in Tunis that targeted a bus carrying members of Tunisia's security forces, killing at least 12 people. We extend our deepest sympathies to the victims' families and have offered assistance with Tunisian authorities' investigation.
"Secretary Kerry was proud to stand with Tunisian leaders earlier this month in Tunis, and reaffirm our countries' extensive economic, governance, and security cooperation. We will continue to support the Tunisian people as they work to build a democratic and prosperous future for their country."
The Tunisian government has condemned the attacks as a 'terrorist act'. However, no terrorist group has claimed responsibility.
Former Tunisian President Moncef Marzouki was the target of an earlier assassination plot, reported Al Jazeera.
The country recently witnessed a terrorist attack at the Bardo National Museum in Tunis. Nearly 21 people were killed, including some European tourists, and around 50 others were injured in the incident.
Islamic State of Iraq and the Levant (ISIL) claimed responsibility, and had warned the country of further attacks.
Tunisia has been under threat from Islamic militants after overthrowing former president Zine El Abidine Ben Ali's government in 2011.