The death toll rises to 28 including elite Revolutionary Guards, in two suicide bomb attacks at a prominent Shi'ite Muslim mosque in the southeast Iranian city of Zahedan Thursday.
100 wounded in the attacks, according to Al Arabiya news report.
The two explosions that took place in front of the mosque on also ripped through nearby shops.
The first explosion occurred at 9:20 p.m. local time (1650 GMT) and was followed by a second blast within minutes, local officials said Thursday.
The Sunni Muslim rebel group Jundollah said it was behind the attacks, telling Al Arabiya television in an e-mail that it had carried them out in retaliation for Iran's execution in June of the group's leader, Abdolmalek Rigi.
Rigi was hanged after being convicted of carrying out other deadly attacks. Jundollah says it is fighting for the rights of Iran's Baluchi Muslim minority.
The suicide bombings took place near Zahedan's Grand Mosque, and Jundollah said they were carried out by relatives of Rigi and were aimed at a Revolutionary Guards gathering, according to Reuters.
cide attacks were carried out by Abdolbaset Rigi and Mohammad Rigi ... and warned of more operations to come," Al Arabiya said.
"In the two explosions in Zahedan more than 20 people were killed and over 100 were injured," Fariborz Rashedi, head of the emergency unit at Sistan-Baluchestan province, told the official IRNA news agency.
It later quoted Zahedan prosecutor Mohammad Marzieh as saying that 21 people had died.
Iran's deputy Interior Minister in charge of security, Ali Abdollahi, said "a number of Iran's Revolutionary Guards were killed and injured," Fars reported.
IRNA said the second attack was so strong that "body parts were scattered around the Grand Mosque."
Predominantly Shi'ite Muslim Iran arrested Rigi in February, four months after Jundollah claimed responsibility for a bombing which killed dozens of people, including 15 members of the Guards. It was the deadliest attack in Iran since the 1980s.
Zahedan is the capital of Sistan-Baluchestan province which shares a border with Pakistan. The province faces serious security problems and there are frequent clashes between police and drug dealers and bandits.
In past, Iran has accused Pakistan, Britain and the United States of backing Jundollah to create instability in southeast Iran.
All three countries have denied this, and Jundollah denies having any links with al Qaeda which Iran claims.
"Confessions of Abdolmalek Rigi prove that America, Israel and some European countries are directly involved in the attacks," said Guards official Yadollah Javadi, Fars reported.
"The enemies of our country try to create conflicts between Shi'ites and Sunnis."
Iran is grappling with ethnic and religious tension in the southeastern province of Sistan-Baluchestan, where authorities have responded to attacks by Baluchi rebels with a spate of hangings. Rights groups and the West have condemned the hangings.
Iran rejects allegations by rights groups that it discriminates against ethnic and religious minorities.