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BAGHDAD, Iraq (CNN) -- A man identified as the leader of al Qaeda in Iraq said on an audiotape Thursday that more than 4,000 foreign fighters have died battling the U.S.-led coalition and Iraqi troops.
"We poured so much of our blood in Iraq," said the tape's speaker, purportedly new al Qaeda in Iraq chief Abu Hamza al-Muhajer.
CNN was unable to verify the speaker's identity. The tape, at 20 minutes, 31 seconds long, was posted on several Islamic Web sites Thursday.
The speaker urged other Muslims in Iraq to join the fight, saying he was launching a major military campaign.
"We are calling upon you to take your responsibilities because we are on crossroads, so don't fail us," the voice said, especially calling on Iraqi professionals such as chemists, physicists and nuclear experts to help.
Al-Muhajer, also known as Abu Ayyub al-Masri, is an Egyptian Islamic militant believed to be an expert in producing car bombs.
He succeeded Abu Musab al-Zarqawi, who died June 7 in a U.S. airstrike north of Baghdad.
The tape's speaker also pushed for the kidnapping of "Christian dogs" who could be exchanged for Omar Abdel-Rahman, an Egyptian Muslim cleric imprisoned since 1995 for conspiring to blow up landmarks in New York.
"I remind you of our dear sheik Omar Abdel-Rahman, who is lying in an American jail undergoing all kinds of abuses -- psychological and physical abuses. I tell him don't be sad. God will bring good news after the hardship," the speaker said.
"I here announce the beginning of a military campaign to uproot the infidels," he said, as Muslims around the world observe the holy month of Ramadan, which continues through late October.
Ramadan, traditionally a time of peace, fasting, soul purification and charity, was called "a month of jihad and martyrdom" on the tape.
The speaker said Sunni Arab tribal leaders who have supported the U.S.-backed government will be granted "amnesty" if they switch over to the insurgency.
"Because Ramadan is the month of forgiveness, we offer the traitor tribal leaders amnesty, on one condition -- that you announce your repenting openly in front of all your people and get the word to us."
In addition, those who repent must cooperate with the mujahedeen to drive the "occupier" out of Iraq, he said.
Earlier this month, al-Muhajer urged each Iraqi Sunni to kill one of the 140,000 U.S. soldiers in Iraq by the start of Ramadan.
Sixty more bodies found in Baghdad
Police found 60 bodies -- all showing signs of torture -- dumped around the Iraqi capital in a 24-hour period ending Thursday morning, pushing the number of bodies discovered so far this week to 122.
Most of the bodies had their hands tied and gunshot wounds to the head, Iraqi emergency police said.
"This has been a tough week," U.S. Maj. Gen. William Caldwell said Wednesday, noting that murders and executions were the largest cause of deaths in Baghdad and attributing them to sectarian strife between Sunnis and Shiites.
Caldwell also reported a rise in insurgent attacks over the past two weeks, particularly in the capital.
"We are seeing an increase in attacks, as anticipated," he said, blaming "terrorists" and "illegal armed groups" for launching strikes during Ramadan.
In Thursday's violence, at least four Iraqis -- including two police officers -- were killed and 38 wounded when a pair of bombs exploded on Saadoun Street, Baghdad's main thoroughfare, Iraqi emergency police said.
The first bomb exploded on the street, and a second one went off as officials rushed to the scene.
In central Baghdad, at least one police officer died and four civilians were wounded when a roadside bomb targeting a police patrol exploded.
Elsewhere in Baghdad, a car bomb killed two Iraqi soldiers and wounded eight others in Sha'ab, a mixed Sunni-Shiite, middle-class neighborhood, police said. Four civilians also were injured.
Also Thursday, 10 police officers were wounded in four separate bomb attacks in the capital.
CNN's Arwa Damon and Octavia Nasr contributed to this report.
Al Qaeda in Iraq leader Abu Hamza al-Muhajer appears in a photo that the U.S. Army released in June.