The chief of Iran's powerful Revolutionary Guard accused the United States, Britain and Pakistan of having links with Sunni militants responsible for a suicide bombing that killed five senior Guard commanders and 37 others.
Iran's president said those behind Sunday's bombing are hiding across the border in Pakistan, and in a phone call with his Pakistani counterpart today he demanded their arrest.
Revolutionary Guard chief Gen Mohammad Ali Jafari vowed to deliver a "crushing" response.
"New evidence has been obtained proving the link between yesterday's terrorist attack and the US, British and Pakistani intelligence services," state TV quoted Jafari as saying.
He said the attack was "undoubtedly" planned and ordered by the three nation's intelligence services and that a delegation would soon travel to Pakistan to present evidence.
The accusations put more strain on the tense relationship that Iran and Pakistan have had for years over the issue of Islamic extremism.
Monday's statements marked the first time Iran has publicly accused its neighbour's intelligence service of supporting the Sunni rebel group known as Jundallah, or Soldiers of God.
Jundallah, which emerged in 2002 in Iran's remote and mountainous southeast, has waged a low-level insurgency there to protest what it says is government persecution of the Baluchi ethnic minority.
Baluchis follow the Sunni branch of Islam and are a minority in predominantly Shiite, Persian Iran.
A claim of responsibility in the name of Jundallah was posted on an Islamic Web site that usually publishes al-Qaida statements. The posting, whose authenticity could not be verified, made no mention of any assistance from foreign powers.
The group has carried out sporadic kidnappings and attacks in recent years - including targeting the Revolutionary Guard and Shiite civilians.
Iran often accuses Western countries, especially the US, of stoking unrest among the country's religious and ethnic minorities - allegations those nations have denied.
Iran has also claimed that Jundallah receives support from al-Qaida and Taliban militants who operate across the border in Pakistan's Baluchistan province, where Baluchi nationalists have been waging a militant campaign for independence from the Pakistani government.
Iran's Jundallah, by contrast, does not appear to seek independence, but rather improved rights for the area's Baluchi people. In 2007, the group adopted a more secular name, the Iranian Popular Resistance Movement, and said it did not depend entirely on armed struggle, but also on political and peaceful efforts to achieve Baluchi rights. The group is still widely referred to by its previous name. AP