Baghdad, Iraq ? Al-Qaida in Iraq’s leader, in a chilling audiotape released Thursday, called for nuclear scientists to join his group’s holy war and urged insurgents to kidnap Westerners so they could be traded for a blind Egyptian sheik who is serving a life sentence in a U.S. prison.

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.

“We are in dire need of you,” said the speaker, who identified himself as Abu Hamza al-Muhajir – also known as Abu Ayyub al-Masri. “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 20-minute audio was posted to a Web site that frequently airs al-Qaida messages. 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 in June north of Baghdad, as head of the al-Qaida-linked organization.

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

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 soon would 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.