A man carrying explosives blew himself up as participants headed to a conference between Shiite and Sunni groups in southeastern Iran on Sunday, killing at least 29 people, CNN reported.
The blast in Sarbaz in the province of Sistan-Baluchistan wounded 28 others, the semiofficial Fars news agency said.
While no one immediately claimed responsibility for the attack, Iran pointed the finger at the United States without disclosing its reasons.
"We consider this recent terrorist act to be the result of the U.S. actions and this is a sign of their enmity," said parliament Speaker Ali Larijani.
A U.S. State Department spokesman said the accusation was "completely false."
"We condemn this act of terrorism and mourn the loss of innocent lives. Reports of alleged U.S. involvement are completely false," Ian Kelly told CNN.
The United Kingdom also condemned the attack.
"The British Government condemns the terrorist attack in the Province of Sistan and Baluchistan in Iran and the sad loss of life which it caused," the Foreign Office said in a statement. "Terrorism is abhorrent wherever it occurs. Our sympathies go to those who have been killed or injured in the attack and their families."
"We reject in the strongest terms any assertion that this was anything to do with the U.K.," the Foreign Office said later in response to reports Iran was accusing London of responsibility.
Five senior officers of Iran's elite Revolutionary Guard Corps were killed in the attack, Fars said. Among the officers was Nour-Ali Shoushtari, the deputy head of the Corps' ground forces, who was in the province to mediate between the two sides, Fars reported.
The province, which borders Pakistan and Afghanistan, has a large minority presence of Baluchis, who are Sunni Muslims.
Other media outlets offered conflicting figures for the number of dead and wounded.
In the past, the predominantly Shiite central government in Tehran has accused the group Jundallah -- also known as the People's Resistance Movement of Iran -- of fomenting unrest in the province. Iran has alleged that the United States and Saudi Arabia are funding the group.
Jundallah says that it is fighting for the rights of Sunni Muslims in the country.
The attack was one of the largest in recent years on the Revolutionary Guard.
Around the same time, a second group of Corps commanders was caught in an explosion when their convoy came under attack at a road intersection between the towns of Sarbaz and Chabahar, state-run Press TV said.
The station did not say whether the second attack resulted in casualties.
The Guard was initially created to protect the leaders of the revolution. But over the years, it has broadened its scope. Today, it is directly under the control of the supreme leader and enforces the government's Islamic codes and morality
With an estimated 200,000-plus members, it is tasked with overseeing the country's crucial interests, including guarding its oil fields and missile arsenals.