The Islamic State terror group claimed responsibility Wednesday for a deadly attack on a bus in central Tunis that was carrying Tunisia's presidential guards.

In a statement posted on the Internet on Wednesday, the group says a militant it identified as Abu Abdullah al-Tunisi carried out the attack after infiltrating the bus and killing around 20 "apostates."

The statement says the "tyrants of Tunis will not have peace and we will not rest until the law of God governs in Tunis."

Tuesday's explosion killed 13 people and the president declared a month-long state of emergency after the attack.

Tunisia's Interior Ministry said 22 pounds of military explosives were used in the blast. It added in a statement Wednesday that either a backpack or an explosive belt containing the explosives was used.

One body recovered in the aftermath is believed to be the "terrorist who caused the explosion," but the interior ministry said the body can't be identified by fingerprints because no fingers were found. A DNA analysis of the body is underway.

The United States condemned the attack Wednesday, with National Security Council spokesman Ned Price saying that the U.S. extends its deepest condolences to the families and loved ones of those killed and wounded.

Price said the U.S. is also prepared to help Tunisian authorities as they investigate the attack. He said terrorists are trying to use fear and violence to undermine important gains the Tunisian people have made in pursuit of a democratic, stable and prosperous country.

The Associated Press contributed to this report.