Mahmoud Ahmadinejad, the Iranian president, said on Sunday that those behind the bombing in the city of Pisheen would be "seriously dealt with".
According to Iranian state media, a Sunni group called Jundallah, or Soldiers of God, claimed responsibility for the attack.
But Tehran has also indicated it believes foreign elements were involved in the attack, the deadliest in Iran in recent years.
"We consider the recent terrorist attack to be the result of US action. This is the sign of America's animosity against our country," Ali Larijani, Iran's parliamentary speaker, said.
Mohammad Marandi, an assistant professor at the University of Tehran, told Al Jazeera the attack could further damage Iran's relations with the US.
"On the one hand, the Americans are talking about rapprochement and building a new future, yet at the same time we see the Americans supporting groups in [Iran's] Kurdish regions as well as in Sistan-Baluchestan."
But Washington has denied involvement with the group, which it has labelled as a "terrorist" organisation, and condemned the attack.
"We condemn this act of terrorism and mourn the loss of innocent lives," Ian Kelly, the US state department spokesman, said in a statement soon after the blast occurred.
"Reports of alleged US involvement are completely false," he said.