Abu Hamza al-Muhajir, also known as Abu Ayyub al-Masri, called for the kidnapping of Westerners to swap for a Muslim jailed in the United States, according to an internet audio tape issued on Thursday.
"I call on every holy fighter in Iraq to strive during this holy month [Ramadan]... to capture some dogs of the Christians so that we can liberate our imprisoned sheikh," he said.
The sheikh referred to in the tape is thought to be Omar Abdel Rahman, an Egyptian man found guilty in 1995 of conspiracy to attack US targets, including the 1993 bombing of the former World Trade Centre in New York.
In the 20-minute tape, al-Masri said that more than 4,000 foreign fighters have died fighting US-led and Iraqi government forces in Iraq.
He also called on scientists to join the fighting in Iraq to develop and test biological or "dirty" weapons.
"We are in dire need of you. The field of jihad can satisfy your scientific ambitions and the large American bases [in Iraq] are good places to test your unconventional weapons, whether biological or dirty," he said.
Al-Masri, an Egyptian said to have trained in Afghanistan and formed al-Qaeda's first cell in Baghdad, became the leader of the network in Iraq after the death of former leader Abu Musab al-Zarqawi in June in a US raid near Baquba, north of Baghdad.
"For those who supported the occupiers and their agents... I declare a general amnesty during this month of generosity and forgiveness"
Abu Ayyub al-Masri,
leader of al-Qaeda in Iraq
In the tape, he offers an amnesty to Iraqis who had co-operated with US-led forces if they repented.
The offer is a reference to Sunni tribal leaders in Iraq's western al-Anbar province who recently said they would unite to fight against al-Qaeda.
"For those who supported the occupiers and their agents... and who betrayed their religion, honour and land for material or social gains ... I declare a general amnesty during this month of generosity and forgiveness," the speaker on the tape said.
The audio recording appeared on a website that often airs tapes and messages from the al-Qaeda network, but its authenticity could not be verified.
A notice posted on Wednesday said that Ayman al-Zawahiri, the deputy head of al-Qaeda, would soon release a new message about the pope, George Bush, the US president, and Sudan's Darfur region.