Pakistan said it would help its neighbour hunt down the culprits. Iran, a mainly Shi'ite Muslim country, says the Sunni rebel group which claimed responsibility for the suicide attack operates out of Pakistan.
Iran's police chief said arrests had been made and that talks were under way with Pakistan about also seizing "the main elements behind the terrorist attack".
"Fortunately, some elements linked to the terrorists have been arrested by the security forces," Esmail Ahmadi-Moghaddam was quoted as saying by the official IRNA news agency.
Iran's Revolutionary Guards, which lost high-ranking officers in the blast, says the militants are backed by the United States and Britain and also have links with Pakistan intelligence. Washington, London and Islamabad deny involvement.
Raising the stakes, a senior Guards commander said on Tuesday his force should be given permission to confront the rebels inside Pakistan, state television reported.
The report did not specify whether General Mohammad Pakpour was referring to authorisation for such an operation from the Pakistani government or from Iranian authorities.
In Islamabad, Pakistan Foreign Minister Shah Mehmood Qureshi declined to comment on the report and said an Iranian delegation was due in Pakistan for talks.
"We will help them and support them in unearthing the people responsible," Qureshi told Reuters on Wednesday. "We will sort this thing out on a government-to-government basis."
He said terrorism was a regional problem and the two countries had to help each other.
"What we are asking is that we as neighbours, as friends, as brotherly friendly countries, have to adopt a cooperative regional approach to deal with this menace," he said. "Pakistan is suffering, Pakistan is a victim of terrorism."
Iranian parliament member Ali Aqazadeh said: "Iran is hoping that Pakistan will take steps ... to extradite the armed rebels to Iran. Otherwise Iran would act against the rebels and their supporting government based upon international norms."
"MAIN ELEMENTS"
Pakistan launched a long-awaited offensive against militants in its northwest on the weekend after a string of bomb and suicide attacks rocked the country in recent weeks and killed more than 150 people.
Analysts say the rebel group Jundollah (God's soldiers) is increasingly inspired by Sunni militants based in Pakistan.
Iranian media say about 42 people died in the bombing in Sistan-Baluchestan province, in Iran's southwest, bordering Afghanistan. Fifteen Guards members were among those killed, including the deputy head of its ground forces. It was the deadliest attack in Iran since its 1980-88 war with Iraq.
The U.N. Security Council, which heeded a request from Tehran and condemned the attack on Tuesday, put the death toll at 57. It was not immediately clear where that figure came from.
The Revolutionary Guards, seen as fiercely loyal to Supreme Leader Ayatollah Ali Khamenei, handle security in border areas. Their power and resources have increased in recent years.
Relations between Iran and Pakistan have been generally good and they are cooperating on plans to build a gas pipeline.
But Iran has in the past accused Pakistan of hosting members of Jundollah, which says it fights for the rights of minority Sunnis in Iran and which has been blamed for many deadly attacks in Sistan-Baluchestan in recent years.
Many Sunnis live in the impoverished desert area, which has seen an increase in bombings and clashes between security forces, ethnic Baluch Sunni insurgents and drug traffickers.
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