Middle East Correspondent Jim Muir: "It is obviously a campaign"

Judicial spokesman Nasiri Savadkuhi was quoted as saying the judicial authorities were "striving to arrest all the perpetrators of the recent mysterious killings, including those responsible for the murder of several writers and journalists".

As soon as they were arrested, they would be punished for their crimes, he said.

He added that investigations were extending beyond the killings in recent weeks of writers and journalists, to include the murder of the head of the Evin Prison, Asadollah Lajevardi, and others.

Five prominent dissidents have been found dead in the past four weeks - three of them writers.

Outside forces

Earlier, Iran's Supreme Leader Ayatollah Ali Khamenei, blamed foreign powers saying "the enemy" was creating insecurity to try to block the progress of Iran's Islamic system.

He vowed to do everything possible to halt the deaths and ordered the security forces to arrest the killers.

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

The president and his fellow moderates are engaged in a continuing power struggle with the conservatives, who are widely regarded as having the sympathy of Ayatollah Khamenei.

President Khatami has pledged to increase democracy and freedom of expression in Iran - reforms which the conservatives fear could undermine their powerbase.

BBC Iranian Affairs Analyst Sadeq Sabah says the reformers fear hardliners are carrying out a new wave of terror to discredit President Khatami's government.

'Sinister pattern'

Human Rights Watch has demanded an investigation into the killings, saying they are part of an "increasingly sinister pattern of harassment and persecution".

Fifty writers have signed an appeal calling on the president to ensure their safety against "a campaign to eliminate freedom in Iran" and disrupt his planned reforms.

The latest body to be found was that of Mohammad Jafar Pouyandeh who vanished on Wednesday. His daughter said he was found strangled on Thursday near the capital, Tehran. None of his belongings had been taken.

Two other liberal writers, Mohammad Mokhtari and Majid Sharif, have died in similar and separate incidents. Another writer, Piruz Davani, is still missing.