Aftermath of the attack in the region of Sistan-Baluchistan
Iran's president has accused Pakistani agents of involvement in a suicide bombing in south-east of the country targeting a group of the elite Revolutionary Guards force.
Mahmoud Ahmadinejad called on Pakistan to arrest the attackers, who he said had entered Iran from Pakistan.
Forty-two people died in the attack in Sistan-Baluchistan, including six Revolutionary Guards commanders.
Iran has previously accused the US and UK of contributing to the attack.
In his first comments on the bombing, Iran's Supreme Leader Ayatollah Ali Khameini also blamed the United States.
"This terrorists crime revealed the evil face of enemies of security and unity who are supported by intelligence organisations of some arrogant governments," he said, using a phrase that usually refers to the US and its allies.
Pakistani officials condemned the attack, which has been blamed on the Sunni resistance group, Jundullah.
The Pakistani foreign ministry spokesman, Abdul Basit, dismissed Iranian claims that Jundullah's leader was in Pakistan.
According to state media, one or more suicide bombers targeted the group of Revolutionary Guards leaders who had arranged to meet tribal leaders in the Pishin district close to the Pakistani border.
Jon Leyne, BBC Tehran correspondent
Jundallah, or Army of God, has been involved in a long-running insurgency in Sistan-Baluchistan.
Some experts believe they may have ties with the Taliban or al-Qaeda, operating in Pakistan and Afghanistan. That is disputed, and another possibility is that Jundallah maintains more informal links with ethnic Baluchis in the neighbouring countries.
Jundallah is also believed to have ties with drugs smugglers, who race across the border in heavily armed convoys, taking drugs on the lucrative smuggling route from Afghanistan through to Western Europe.
Reports said a suicide bomber detonated a belt packed with explosives as the meeting was about to start.
The deputy commander of the Guards' ground force, General Noor Ali Shooshtari, and the Guards' chief provincial commander, Rajab Ali Mohammadzadeh, were among at least six officers reported to have been killed.
Dozens of people were injured.
A top Guards officer has also vowed to deliver a "crushing" response to those behind the attack, according to Agence-France Presse.
Mr Ahmadinejad pointed towards Pakistan.
"We were informed that some security agents in Pakistan are co-operating with the main elements of this terrorist incident," he was quoted as saying by the semi-official Fars news agency.
"We regard it as our right to demand these criminals from them," he said, without elaborating.
"We ask the Pakistani government not to delay any longer in the apprehension of the main elements in this terrorist attack."
Iran's Revolutionary Guard have been hit in the country's south-east before
Iranian authorities summoned a senior Pakistani diplomat in Tehran, claiming that the assailants had arrived in Iran from Pakistan, Iranian state media reported.
The Iranian foreign ministry also "protested against the use of Pakistani territory by the terrorists and rebels against the Islamic Republic of Iran", the Isna news agency reported.
Earlier, Parliament Speaker Ali Larijani said "US action" contributed to the attack. The US dismissed the accusation.
Banditry and kidnapping
Sistan-Baluchistan is mainly made up of the Baluchi ethnic group, who belong to the Sunni Muslim minority of Shia-ruled Iran.
May 2009 A bomb explodes at a mosque in the provincial capital Zahedan, killing 19 and injuring 60.
February 2007 Suspected militants killed 11, including Iranian Revolutionary Guards, in a bomb attack in the provincial capital Zahedan.
March 2006 Gunmen posing as police kill 22 people, many government employees, after closing the Zabol-Zahedan road.
Jundallah has previously been accused by Iran of terrorist activities in the province.
The province borders both Pakistan and Afghanistan, and has long been affected by smuggling, drug trafficking, banditry and kidnapping.
Jundallah, also known as the Popular Resistance Movement of Iran, says it is fighting against the political and religious oppression of the country's minority Sunni Muslims.
Local media cited officials as saying Jundallah had accepted responsibility for Sunday's bombing, though there was no direct claim from the group.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.