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(CNN) -- The leader of al Qaeda in Iraq urged his followers to kill at least one American in the next two weeks using a sniper rifle, explosive or "whatever the battle may require," according to an audiotape that aired Thursday on Al-Jazeera.
The Arabic news network identified the man on the tape as Abu Hamza al-Muhajer, however CNN was unable to independently verify his identity.
Al-Muhajer took command of the terror network's partner group in Iraq after an American airstrike killed its former leader, Abu Musab al-Zarqawi, in June, U.S. officials say.
Al-Muhajer, which means "the immigrant," is the pseudonym adopted by Abu Ayyub al-Masri, an Egyptian militant believed to be an expert at making car bombs, U.S. officials say.
"I invite you not to drop your weapons, and don't let your souls or your enemies rest until each one of you kills at least one American within a period that does not exceed 15 days with a sniper's gunshot or incendiary devices or Molotov cocktail or a suicide car bomb -- whatever the battle may require," the speaker says on the tape.
The recording is more sophisticated than past tapes released by al Qaeda in Iraq and includes sound effects such as a lion's roar, machine gun fire and a sword being unsheathed.
Producers at times used echo effects on al-Muhajer's voice and included snippets of inspirational songs and verses from the Quran.
On the tape, al-Muhajer blasts Iraq's Sunni Muslim community for cooperating with the Shiite-led and American-backed Iraqi government. He calls for unity among Muslims, "so the word of God can be the highest, and the word of the infidels the lower."
"Our enemy has united its sides against us, and isn't it time to unite, you worshippers of God?" al-Muhajer asks.
The full statement is more than 18 minutes long, and al-Muhajer is introduced in a voice similar to one that introduced al-Zarqawi in previous tapes, said counterterrorism expert Laura Mansfield.
There is no way to tell for certain if the speaker is al-Muhajer because there is no previous recording of his voice to compare it with. But there are "striking similarities" between Thursday's tape and other tapes released by al Qaeda in Iraq shortly after al-Zarqawi's death, she said.
Shootings, blasts kill at least 20
Insurgent attacks ripped across Baghdad on Thursday. At least 20 people were killed and 24 bodies were found dumped in the capital -- killings that bore the hallmarks of sectarian violence.
The bodies were found in various neighborhoods, police said. The dumping of these remains has been linked with Sunni-Shiite retribution killings.
In the bloodiest attack, at least 10 people were killed and several others were wounded when a suicide car blast occurred near Iraqi emergency police headquarters in the southeastern neighborhood of Alwiya, emergency police said.
Three vehicles and nearby shops were damaged in the bombing. Officers were refueling their vehicles in an industrial area behind the police station when the attack took place, police said.
In northern Baghdad, three people were killed and 13 others wounded when a car bomb exploded Thursday morning near a joint police and army patrol in the Qahira neighborhood, a Baghdad emergency police official said. (Watch what happens on a deadly day in Baghdad -- 1:35)
A suicide car bomb also detonated outside a fueling station close to the Iraqi Interior Ministry complex in central Baghdad's Bab al-Shurji district, police officials said. Two police officers were killed and 23 other people were wounded, including six police officers, a source said.
In southern Baghdad's Dora neighborhood, two police officers were killed in a drive-by shooting. Three police officers were wounded when two roadside bombs exploded there.
In western Baghdad, a civilian was killed and four police officers were wounded when a roadside bomb struck a police patrol. Also in that part of the capital, gunmen killed a doctor while he was on his way to work at Yarmouk Hospital.
In southwestern Baghdad's neighborhood of Amil, a roadside bomb killed a civilian and wounded 13 others.
In addition, gunmen Wednesday night kidnapped the nephew of Iraqi parliament Speaker Mahmoud Al-Mashhadani in northwestern Baghdad. Ahmad Al-Mashhadani was abducted in the Hurriya neighborhood, police said.
Earlier Wednesday, al-Mashhadani, a Sunni Arab, had warned colleagues in parliament that Iraq will collapse by year's end if warring groups fail to reconcile.
• The Iraqi government Thursday shut down the Baghdad offices of Al-Arabiya, a popular Arabic television station, Al-Arabiya spokesman Najib Ben Cherif said. Authorities did give a reason, he said, only that the government had ordered the offices closed for a month. The Baghdad bureau chief, Jawad al-Hattab, said he learned about the closure from Iraq State TV, which said Prime Minister Nuri al-Maliki's office issued the order.
• The Iraqi government has begun taking operational control of its military from the U.S.-led coalition. A handoff ceremony was held Thursday, naming al-Maliki as commander in chief. Iraq's air and naval forces and one of its 10 army divisions -- about 10 percent of the military -- now are under Iraqi control. More units are expected to be transferred at a rate of up to two divisions a month.
• Two U.S. soldiers and a Marine were killed Wednesday in Iraq, the U.S. military said Thursday. One soldier was shot and killed "while executing a mission" in northern Iraq. A second died from "injuries sustained from enemy action" in Anbar province. The Marine died of "wounds sustained from enemy action," in Anbar, the military said. The number of U.S. troops killed stands at 2,657 since the 2003 invasion of Iraq.
CNN's Mohammed Tawfeeq contributed to this report