Shiite extremists launched several rockets toward the heavily fortified Green Zone on Friday, taking advantage of a sudden sandstorm and low visibility that gave cover from attack by U.S. aircraft. Some of the rockets fell short.

One rocket hit the roof of a building housing the British Broadcasting Corporation's Baghdad bureau, damaging equipment but causing no injuries.

At least five other rocket explosions were heard but U.S. authorities did not confirm any impacts in the Green Zone that houses the U.S. Embassy and much of the Iraqi government on the west side of the Tigris River.

U.S. and Iraqi forces are building a barrier — reaching up to 12-feet high — to isolate Shiite extremists in the southern part of the Sadr City district, from where rocket or mortar strikes usually originate.

The "rocket struck the roof of the BBC Baghdad bureau. It caused structural damage but no one was injured," Patrick Howse, the BBC bureau chief in Baghdad said. He said it left a big hole in the roof, approximately 3.2 feet by 4.9 feet.

U.S. authorities plan to complete the wall within two weeks. The wall, once finished, will cut off militias from outside world and enable the military to exert more control over the most troubled section of Sadr City.

The U.S. military on Friday said six Shiite extremists, who attacked U.S. forces with shoulder fired rockets and small arms in Sadr City, have been killed. The clashes occurred Thursday.

At least four people were killed and 51 others wounded in Sadr City over the past 24 hours, Iraqi health officials said Friday. It was not clear if any militants were among them.

A parked car bomb exploded Thursday in the capital's western neighborhood of Harthiya injuring five civilians, a police official, speaking on condition of anonymity because he was not authorized the speak to the media, said.

The street battles in Sadr City between U.S.-backed Iraqi forces and extremists who broke away from anti-American cleric Muqtada al-Sadr's powerful Mahdi Army militia have killed or displaced thousands of people.

The clashes erupted after Prime Minister Nouri al-Maliki, a Shiite, launched a crackdown against the Mahdi Army in the southern city of Basra on March 25. He has vowed to disarm the Mahdi Army and other outlawed groups in Iraq as the crisis tested his rule.

Government envoys set strict demands for Shiite militias to end their battles against U.S.-led forces in Baghdad in meetings Thursday, but it was unlikely that militiamen would abide by the conditions and lay down arms.

"There is no dictator in the world who did what al-Maliki is doing against his people," Sadrist cleric Abdul-Hadi al-Mohammedawi told worshippers Friday in the southern city of Kufa.

Al-Sadr last month threatened to unleash an open-war against the U.S.-led forces but ordered the militiamen to avoid spilling Iraqi blood.

Another enemy, Al Qaeda in Iraq announced April 19 that it was launching a one-month offensive against U.S. troops and U.S.-allied Sunnis.

The U.S. military on Friday denied that the leader of Al Qaeda in Iraq, Abu Ayyub al-Masri, had been captured, saying a man with a similar name had instead been arrested in the northern city of Mosul.

"Neither coalition forces nor Iraqi security forces detained or killed Abu Ayyub al-Masri. This guy had a similar name," said Maj. Peggy Kageleiry, a U.S. military spokeswoman in northern Iraq.

Iraqi Defense Ministry spokesman Mohammed al-Askari said the confusion arose because the commander of Iraqi forces in northern Ninevah province was convinced that he had arrested al-Masri — also known as Abu Hamza al-Muhajir.

There have been false alarms in the past about al-Masri. At least twice — in 2006 and May 2007 — reports circulated that he was dead, but they were later proved wrong.

"Iraqi officials are dealing with a developing chain of command that often leaps to conclusions and reports success before it occurs, often under pressure from the media," Anthony Cordesman, a security analyst for the Center for Strategic and International Studies said on Friday.

Al-Masri took over Al Qaeda in Iraq after its leader, Abu Musab al-Zarqawi, was killed June 7, 2006 in a U.S. airstrike northeast of Baghdad.

U.S. officials said al-Masri — whose name means "The Egyptian" in Arabic — joined al-Qaeda camps in Afghanistan in the late 1990s and trained as a car bombing expert before traveling to Iraq after the U.S.-led invasion in 2003. The U.S. military has a US$5 million (euro3.25 million) bounty for al-Masri.

U.S. spokesman Rear Adm. Patrick Driscoll vowed last Sunday that there was "no place for Al Qaeda" amid heightened concerns the organization is regrouping after suffering a serious blow last year when thousands of Sunni tribesmen turned against the terrorist group, which is blamed for most of Iraq's car bombings and homicide attacks.

Cordesman, however, said Al Qaeda in Iraq has "five years of experience in avoiding" surveillance by U.S.-led or Iraqi forces.

"They have developed a wide range of evasion tactics and have sharply reduced the intelligence indicators the U.S. and others can use for their top leaders — distancing them from communications, combat and all forms of visibility," Cordesman said. "It normally takes a human intelligence breakthrough to target a key leader — who may not even be in the country — or sheer luck from a related raid."

He said Al Qaeda in Iraq will remain "a serious threat" until a solid base of Sunni-Shiite political accommodation was formed and the economy and security are improved in Sunni areas to "deprive Al Qaeda in Iraq of a recruiting base."

"Al Qaeda in Iraq can carry out large-scale bombings and suicide attacks with a very small number of cadres, and as long as Iraqi Sunni-Shiite tensions remain serious, such attacks can trigger sectarian fighting," Cordesman said.