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A rocket landed near the prime minister's office Thursday during the first visit to Iraq by the head of the United Nations in nearly a year and a half, sending Secretary-General Ban Ki-moon ducking unharmed behind a podium at a news conference.

The rocket caused no injuries but rattled the building in the heavily guarded Green Zone, sent small chips of debris floating from the ceiling, and left a three-foot-wide crater about 50 yards away outside. Prime Minister Nouri al-Maliki told his security guards "Nothing's wrong," as they moved to grab him, and the news conference kept going.

The explosion occurred right after al-Maliki had finished telling reporters that Ban's visit was a sign that Iraq was on the road to stability.

"We consider it a positive message to (the) world in which you confirm that Baghdad has returned to playing host to important world figures because it has made huge strides on the road toward stability," al-Maliki said in his opening remarks.

The last visit to Iraq by the head of the U.N. was in November 2005, by Ban's predecessor, Kofi Annan.

The United Nations headquarters in Baghdad was bombed by militants on Aug. 19, 2003, and 22 people died, including the top U.N. envoy, Sergio Vieira de Mello. The U.N.'s international staff withdrew from Iraq in October 2003 following a second assault on its offices and other attacks on humanitarian workers. A small staff has gradually been allowed to return since August 2004.

Ban lauded al-Maliki's government's performance and promised continued U.N. assistance with reconstruction and security, but he appeared to criticize it for not doing enough to broaden the political process.

"I had hoped that the Iraqi government...will engage in an all-inclusive political process," he said. "I also hope that countries in the region will constructively engage in helping the Iraqi government and people."

The attack came as al-Maliki's government said it had been negotiating with Sunni insurgents for months but remained deadlocked over demands for a timetable for a U.S. troop withdrawal.

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The U.S. military said that it had released a senior aide to Shiite cleric Muqtada al-Sadr on al-Maliki's request because it believed that the peace process could be helped by Ahmed al-Shibani. He was captured in the holy Shiite city of Najaf during fierce clashes in 2004 between U.S. forces and al-Sadr's Mahdi Army militia, which has largely cooperated with a new security push by U.S. and Iraqi forces.

The government released a photograph showing al-Maliki receiving a smiling al-Shibani at his office, underlining the close ties between the prime minister and al-Sadr. Al-Sadr's support won al-Maliki his job last year, since the cleric's loyalists have 30 of parliament's 275 seats.

The U.S. military said it had determined that al-Shibani "could play a potentially important role in helping to moderate extremism and foster reconciliation in Iraq."

Authorities imposed an indefinite curfew on the southern city of Basra after clashes between the Mahdi Army and the rival Shiite Fadhila party, which recently withdrew from al-Maliki's government.

The continuing rivalry in the oil-rich region poses the most serious threat to the unity of Iraq's majority Shiites.

Police said the Mahdi militiamen fought Fadhila guards outside party headquarters, capturing eight before the building caught fire. Twelve Mahdi Army fighters were wounded, they said.

Clashes also erupted near the residence of Basra's Fadhila governor, Mohammed al-Waeli, and continued into the afternoon, police said.

The clashes came days after British forces pulled out of their main base in the heart of Basra.

The U.S. military announced that it had captured the leaders of a Shiite insurgent network "directly connected" to the killing in January of five American soldiers in the holy city of Karbala by gunmen speaking English, wearing U.S. military uniforms and carrying American weapons.

The military said the arrests of Qais al-Khazaali, his brother Laith al-Khazaali and several other members of the network took place over the past three days.

On Wednesday, two senior Mahdi Army commanders told The Associated Press that Qais al-Khazaali was the leader of up to 3,000 fighters who defected from the militia and were now financed directly by Iran and no longer loyal to firebrand cleric Muqtada al-Sadr.

Qais al-Khazaali, a cleric in his early 30s, was a close aide to al-Sadr in 2003 and 2004. He served as al-Sadr's spokesman during the 2004 fighting in Najaf, appearing daily on Arabic satellite news channels. The outspoken al-Khazaali has not been seen in public since late 2004.

Saad Yousif al-Muttalibi of the Ministry of National Dialogue and Reconciliation, meanwhile, told the AP in a telephone interview that talks with Sunni insurgent groups were initiated at the request of the insurgents and have been taking place inside and outside Iraq over the past three months.

He refused to identify the groups, but said they did not include Al Qaeda in Iraq or Saddam Hussein loyalists. Members of the former president's outlawed Baath party took part, he added.

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Al-Muttalibi said the negotiations were deadlocked over the insurgent groups' insistence that they would lay down their arms only when a timetable for the withdrawal of U.S.-led coalition troops in Iraq is announced.

The government's response was that such a move could only be taken when security is restored.

Future rounds of negotiations are planned, he said, but did not elaborate.

The U.S. military reported that two soldiers and a Marine were killed in combat on Wednesday. One soldier was killed in Baghdad; a second soldier and a Marine perished in Anbar province, the Sunni insurgent stronghold west of Baghdad.

The clashes in Basra, 340 miles southeast of Baghdad, followed the arrest of a Mahdi Army militiaman by policemen thought to be loyal to Fadhila, and the retaliatory kidnapping and killing of a Fadhila supporter, according to police.

In the volatile city of Baqouba northeast of Baghdad, the bullet-ridden body of a kidnapped local official and mother of three was found dumped on a city street, one day after masked gunmen stormed her house and took her away handcuffed, police said.

Ilham Namik Shahin, 43, was a Shiite member of the Baqouba provincial council. Her brother, Najah Namik Shahin, said 10 gunmen stormed the family home Wednesday night, ordered everyone into the living room before they handcuffed his sister and left with her.

"It took just 10 minutes and we were really scared. We couldn't talk," said the brother.

Baqouba, in Diyala province 35 miles northeast of Baghdad, has seen an upsurge in violence and sectarian killings in recent weeks, with Sunni insurgents loyal to Al Qaeda in Iraq stepping up attacks as violence appears to ebb in Baghdad since the security push began Feb.14.

Also in Basra, police said gunmen on a motorcycle shot dead a postgraduate female student at Basra University, Tuhfa Jaafar al-Bachay, Wednesday night outside her home.

Like most killings that take place daily in Iraq, the motive for al-Bachay's murder was unknown.

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