(Adds background on Muhajir)

DUBAI, May 9 (Reuters) - The leader of al Qaeda in Iraq, Abu Hamza al-Muhajir, has been arrested, the Arabic television station al-Arabiya reported on Friday, quoting the Iraqi Defence Ministry.

Arabiya said Muhajir had been detained in a joint Iraqi-U.S. operation in the northern Iraqi city of Mosul. The U.S. military said it had no information on the reports at this stage.

Al Qaeda in Iraq was headed by the Jordanian militant Abu Musab al-Zarqawi until he was killed in a U.S. air strike in June 2006.

His successor, Muhajir, an Egyptian also known as Abu Ayyab al-Masri, was Zarqawi's close associate, and has a U.S. bounty of $5 million on his head.

In October 2006, the al Qaeda-led Mujahideen Shura Council said it had set up the Islamic State of Iraq, an umbrella group of Sunni militant affiliates and tribal leaders led by Abu Omar al-Baghdadi. In April 2007 it named a 10-man "cabinet", including Muhajir as its war minister.

Iraq's Interior Ministry said last May that Masri had been killed, but soon afterwards al Qaeda released an audio tape purportedly from him.

In an hour-long audio tape issued last month, Muhajir called for renewed attacks on American troops.

He urged militants from the Sunni Islamist group to "celebrate" the recent announcement that the number of U.S. troops killed in Iraq had passed 4,000.

"We must celebrate this event in our special way, and make the defeated Bush join us in this celebration," he said.

He called on al Qaeda fighters to provide "a head of an American as a present to the trickster Bush" in a month-long campaign that he called the "Attack of Righteousness".

Al Qaeda in Iraq shares a name and ideology if not organisational ties with Osama bin Laden's network, which was blamed for the Sept. 11, 2001 attacks on the United States. (Reporting by Inal Ersan; Editing by Kevin Liffey)

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