Tehran, Iran(CNN) A suicide bombing hit a bus carrying members of the Iranian military's elite Revolutionary Guard in the southeastern province of Sistan-Baluchestan, killing at least 23 and wounding 17, according to the semiofficial Fars News Agency.
Iranian state media has offered differing death tolls for the attack, ranging from 27 to 41.
A separatist group called Jaish al-Adl, or Army of Justice, has claimed responsibility, according to state-run news agency IRNA.
The incident took place on a desert road near Iran's volatile border with Pakistan and Afghanistan, where the group is known to operate. It comes two days after Iran marked the 40th anniversary of the Islamic Revolution.
The truck was loaded with explosives when it detonated and struck the bus carrying members of the guard who were coming back from "a border guarding mission," Fars reported.
Foreign Ministry spokesman Bahram Qassemi said Iran will seek revenge for the suicide attack, according to the news agency.
"The self-sacrificing military and intelligence children of the people of Iran will take revenge for the blood of the martyrs of this incident," Qassemi said.
Ali Fadavi, a commander Iran's Islamic Revolution Guards Corp, has promised a strong response to the deadly bombing, Fars reported.
"Our response, aimed at safeguarding the Islamic Revolution, shall not be limited only to the geography which pertains just to us," Fadavi said.
The bombing happened as world leaders gathered in Warsaw, Poland, for a meeting with US Secretary of State Mike Pompeo. The meeting has been billed as a conference to counter Iran's military expansion.
Iran is embroiled in several regional conflicts, including those in Syria and Yemen.
Nearly a year after President Donald Trump pulled the United States out of the landmark Iran nuclear deal, the country's economy has buckled under aggressive US sanctions.
Prices have soared, the currency has plummeted and food and medical shortages have affected households across the country.