BAGHDAD, Iraq (AP) – Chemists. Physicists. Nuclear scientists. Explosives experts.

“We are in dire need of you,” al-Qaida in Iraq’s leader said in a chilling audio message released Thursday. “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, as they call them.”

The fugitive terror chief said experts in the fields of “chemistry, physics, electronics, media and all other sciences – especially nuclear scientists and explosives experts” should join his group’s jihad, or holy war, against the West.

The speaker on the tape identified himself as Abu Hamza al-Muhajir – also known as Abu Ayyub al-Masri. The voice could not be independently identified, but it was thought to be al-Masri’s.

He is believed to have succeeded Abu Musab al-Zarqawi, who died in a U.S. airstrike north of Baghdad in June, as head of the al-Qaida-linked organization.

In the 20-minute audio – posted to a Web site that frequently airs al-Qaida messages – al-Masri, an Egyptian, also urged insurgents to kidnap Westerners so they could be traded for a blind, 68-year-old Egyptian cleric who is serving a life sentence in a U.S. prison.

The cleric, Sheik Omar Abdel-Rahman, was convicted in 1995 of seditious conspiracy for his advisory role in a plot to assassinate Egypt’s president and blow up five New York City landmarks including the United Nations. Abdel-Rahman is considered the leader of Egyptian Islamic militants, and the 1993 World Trade Center conspirators were known to have attended his lectures.

“I appeal to every holy warrior in the land of Iraq to exert all efforts in this holy month so that God may enable us to capture some of the Western dogs to swap them with our sheik and get him out of his dark prison,” al-Masri said.

Al-Masri also said more than 4,000 foreign militants have been killed in Iraq since the U.S.-led invasion in 2003 – the first known statement from the insurgents about their death toll.

It was unclear why al-Masri would advertise the loss of the group’s foreign fighters, but martyrdom is revered among Islamic fundamentalists, and could be used as a recruiting tool. Analysts said the announcement was likely a boast aimed at drumming up support.

“It’s showing the level of dedication to their cause, the level of sacrifice jihadists are making,” said Ben N. Venzke, director of the Washington-based IntelCenter, which monitors terrorism communications.

“In a strange kind of way, it’s almost showing a sense of strength and purpose in their cause to other people around world who might be thinking about joining the fight,” Venzke said in a telephone interview.

The statement followed the release of a U.N. report Wednesday that said fewer foreign fighters have been killed or captured in Iraq in the last few months, “suggesting that the flow has slackened.” The report also said some fighters had expressed dissatisfaction they were asked to kill fellow Muslims rather than Western soldiers and that the only role for them was to be suicide bombers.

Still, the report said al-Qaida “has gained by continuing to play a central role in the fighting and in encouraging the growth of sectarian violence; and Iraq has provided many recruits and an excellent training ground.”

On the tape, al-Masri offered amnesty to Iraqis who cooperated with their country’s “occupiers,” calling on them to “return to your religion and nation” during the Muslim holy month of Ramadan, which Sunnis began observing in Iraq on Saturday and Shiites on Monday.

“We will not attack you as long as you declare your true repentance in front of your tribe and relatives,” he said. “The amnesty ends by the end of this holy month.”

The audio message came on a day that saw the killings of at least 23 people and the discovery in the capital of 40 apparent victims of sectarian death squads. To stem the violence, the government announced it will soon lock down traffic access to Baghdad.

Thursday’s attacks in the capital included a car bombing that killed five and wounded 34 near a restaurant in the city’s center, and a suicide car bombing on a military checkpoint that killed two Iraqi soldiers and wounded 10.

One person was killed and 24 were wounded in two mortar attacks on residential areas in northern Baghdad.

The 40 bodies found all showed signs of torture, had been shot, and had their hands and feet bound, police Lt. Thayer Mahmoud said.

The violence comes amid reports from senior coalition military officials that a militia run by radical Shiite cleric Muqtada al-Sadr has been breaking apart into freelance death squads and gangs.

Al-Sadr’s Mahdi Army is one of the largest and most powerful militias in Iraq, along with the Badr Brigades – which was once the military wing of the country’s largest Shiite political group, the Supreme Council for the Islamic Revolution in Iraq.

“There are fractures politically inside Sadr’s movement, many of whom don’t find him to be sufficiently radical now that he has taken a political course of action,” a senior coalition intelligence official told reporters in Baghdad, speaking on condition of anonymity because he was not permitted to speak publicly on intelligence issues.

The official added that “I can think of about at least six major players who have left his organization because he has been perhaps too accommodating to the coalition.”

Last Friday, al-Sadr urged his followers not to use force against U.S. troops, saying “I want a peaceful war against them and not to shed a drop of blood.

As part of the Baghdad security plan crackdown on sectarian militias, known as Operation Together Forward, U.S. and Iraqi troops have been going neighborhood-by-neighborhood clearing buildings.

The operation has not yet reached Sadr City, a sprawling Shiite slum of about 2 million where al-Sadr draws much of his support, but it will not be left out, the intelligence official said.

“The Baghdad security plan will cover Baghdad,” the official said. “I didn’t say Baghdad minus; I said Baghdad.”

Government spokesman Ali al-Dabagh indicated that another part of the plan would begin soon: funneling all vehicular traffic into Baghdad through 28 checkpoints.

“The gaps between natural barriers such as Tigris river and canals will be filled with artificial barriers in order to control and observe any threats against Iraqis,” he said. “This thing would lead to traffic jams for people entering Baghdad, we hope that our people will understand the reasons behind this act designed to protect them.”