In remarks reported by the Iranian news agency, the official, Reza Amini, said that the "the domestic and external hands" behind a spate of mysterious killings in Tehran would be tracked down.

Mr Amini is an adviser to Iran's most senior judge Ayatollah Mohammad Yazdi, who has rejected calls for an international investigation into the killings.

Mr Amini said the killings bore the hallmarks of two armed opposition groups active in the early days of the Islamic revolution - Forqan and the followers of the late Mehdi Hashemi - and he was certain the killings were backed from abroad.

Ayatollah Yazdi rejects calls for international investigation (in Farsi)

At Friday prayers in Tehran, Ayatollah Yazdi told worshippers that "a number of people" were under arrest in connection with the killings, and the investigation had reached "a semi-clear and solid point."

He said calls for an international investigation into the killings, were part of an "enemy" campaign to discredit Iran and interfere in its affairs.

Ayatollah Yazdi: Results soon

"As you can see, in Algeria, brutal slaughters have been going on, even recently and even in Ramadan," he said. "However, there has been no talk of sending a UN investigation team there."

Several Iranian dissidents have died in mysterious circumstances since November and the authorities have come under growing pressure to produce some results in their investigation.

Some international human rights organisations have asked that they be allowed to dispatch fact-finding teams to Iran to investigate the killings.

Hardliners suspected

Correspondents say the main suspects are hardliners opposed to Iran's reformist President Mohammad Khatami.

President Khatami and his fellow moderates are engaged in a continuing power struggle with conservatives, who are widely regarded as having the sympathy of Iran's Supreme Leader Ayatollah Ali Khamenei.

Ayatollah Yazdi said senior officials, including the president and Ayatollah Khamenei, had "issued hard and explicit orders for the matter to be investigated, clarified, and its perpetrators found and punished and further incidents of this kind prevented."