Iran's chief prosecutor says perpetrators of a deadly bomb blast at a religious center in Shiraz have admitted to having links to the occupying Israeli regime and the US.

Ghorbanali Dorri-Najafabadi said that those responsible for the attack have confessed to having links to "worldwide oppression, in particular the United States and Israel".

They also admitted carrying out "one or two minor operations," the the ISNA news agency quoted him as saying, adding the group launched military operations a year ago.

Iran has arrested 15 people over the explosion and accused the US and Britain of training and financing those behind the April 12 bombing which killed 14 people and wounded more than 200.

Tehran Friday prayers leader Ayatollah Ahmed Khatami said in his sermon that those arrested had also plotted attacks in the holy city of Qom, 120 kms (75 miles) south of Tehran, and at a book fair held in the capital.

Iran protested to the US on Wednesday over giving haven to a terrorist group which has been behind a series of sabotage attacks in Iran.

The protest was lodged through the Swiss embassy, which represents US interests in Iran in the absence of diplomatic ties.

The Swiss charge d'affaires was summoned to the foreign ministry and was informed of Iran's strong protest against free activities of the terrorist group in the US.