The leader of al-Qaeda in Iraq, Abu Hamza al-Muhajir, has called for kidnapping of Westerners, according to a recording issued on the internet.
"I call on every holy fighter in Iraq to strive to capture some dogs of the Christians so that we can liberate our imprisoned sheikh," the tape says.
It is a reference to Egyptian cleric Omar Abdul Rahman, held in the US over the 1993 World Trade Center bombing.
Issued on Islamist websites, the tape's authenticity could not be verified.
It appears to have been issued to mark the beginning of the Islamic holy month of Ramadan, normally a time for prayer and fasting.
"I congratulate the Muslim nation on the occasion of the holy month of Ramadan, the month of jihad (holy war).
"I ask God to make it a month for honour and victory for Muslims," the voice says.
The recording discloses for the first time the number of non-Iraqi insurgents said to have been killed fighting US-led forces in Iraq since the overthrow of Saddam Hussein in 2003.
"More than 4,000 emigres and many more of the supporters of righteousness [Iraqi insurgents] have given their blood to Iraq," the speaker says.
It also calls for explosives experts and nuclear scientists to help launch non-conventional attacks on US bases, using biological or radialogical weapons, so-called "dirty bombs".
The recording also offers an amnesty to those who had collaborated with US-led forces or fled the country, if they repented before the end of Ramadan.